[PROBLEM]
The existing scrub code for data extents always limit the block size to
sectorsize.

This causes quite some extra scrub_block being allocated:
(there is a data extent at logical bytenr 298844160, length 64KiB)

  alloc_scrub_block: new block: logical=298844160 physical=298844160 mirror=1
  alloc_scrub_block: new block: logical=298848256 physical=298848256 mirror=1
  alloc_scrub_block: new block: logical=298852352 physical=298852352 mirror=1
  alloc_scrub_block: new block: logical=298856448 physical=298856448 mirror=1
  alloc_scrub_block: new block: logical=298860544 physical=298860544 mirror=1
  alloc_scrub_block: new block: logical=298864640 physical=298864640 mirror=1
  alloc_scrub_block: new block: logical=298868736 physical=298868736 mirror=1
  alloc_scrub_block: new block: logical=298872832 physical=298872832 mirror=1
  alloc_scrub_block: new block: logical=298876928 physical=298876928 mirror=1
  alloc_scrub_block: new block: logical=298881024 physical=298881024 mirror=1
  alloc_scrub_block: new block: logical=298885120 physical=298885120 mirror=1
  alloc_scrub_block: new block: logical=298889216 physical=298889216 mirror=1
  alloc_scrub_block: new block: logical=298893312 physical=298893312 mirror=1
  alloc_scrub_block: new block: logical=298897408 physical=298897408 mirror=1
  alloc_scrub_block: new block: logical=298901504 physical=298901504 mirror=1
  alloc_scrub_block: new block: logical=298905600 physical=298905600 mirror=1
  ...
  scrub_block_put: free block: logical=298844160 physical=298844160 len=4096 mirror=1
  scrub_block_put: free block: logical=298848256 physical=298848256 len=4096 mirror=1
  scrub_block_put: free block: logical=298852352 physical=298852352 len=4096 mirror=1
  scrub_block_put: free block: logical=298856448 physical=298856448 len=4096 mirror=1
  scrub_block_put: free block: logical=298860544 physical=298860544 len=4096 mirror=1
  scrub_block_put: free block: logical=298864640 physical=298864640 len=4096 mirror=1
  scrub_block_put: free block: logical=298868736 physical=298868736 len=4096 mirror=1
  scrub_block_put: free block: logical=298872832 physical=298872832 len=4096 mirror=1
  scrub_block_put: free block: logical=298876928 physical=298876928 len=4096 mirror=1
  scrub_block_put: free block: logical=298881024 physical=298881024 len=4096 mirror=1
  scrub_block_put: free block: logical=298885120 physical=298885120 len=4096 mirror=1
  scrub_block_put: free block: logical=298889216 physical=298889216 len=4096 mirror=1
  scrub_block_put: free block: logical=298893312 physical=298893312 len=4096 mirror=1
  scrub_block_put: free block: logical=298897408 physical=298897408 len=4096 mirror=1
  scrub_block_put: free block: logical=298901504 physical=298901504 len=4096 mirror=1
  scrub_block_put: free block: logical=298905600 physical=298905600 len=4096 mirror=1

This behavior will waste a lot of memory, especially after we have moved
quite some members from scrub_sector to scrub_block.

[FIX]
To reduce the allocation of scrub_block, and to reduce memory usage, use
BTRFS_STRIPE_LEN instead of sectorsize as the block size to scrub data
extents.

This results only one scrub_block to be allocated for above data extent:

  alloc_scrub_block: new block: logical=298844160 physical=298844160 mirror=1
  scrub_block_put: free block: logical=298844160 physical=298844160 len=65536 mirror=1

This would greatly reduce the memory usage (even it's just transient)
for larger data extents scrub.

For above example, the memory usage would be:

Old: num_sectors * (sizeof(scrub_block) + sizeof(scrub_sector))
     16          * (408                 + 96) = 8065

New: sizeof(scrub_block) + num_sectors * sizeof(scrub_sector)
     408                 + 16          * 96 = 1944

A good reduction of 75.9%.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>