Currently btrfs uses fixed stripe length (64K), thus u32 is wide enough
for the usage.

Furthermore, even in the future we choose to enlarge stripe length to
larger values, I don't believe we would want stripe as large as 4G or
larger.

So this patch will reduce the width for all in-memory structures and
parameters, this involves:

- RAID56 related function argument lists
  This allows us to do direct division related to stripe_len.
  Although we will use bits shift to replace the division anyway.

- btrfs_io_geometry structure
  This involves one change to simplify the calculation of both @stripe_nr
  and @stripe_offset, using div64_u64_rem().
  And add extra sanity check to make sure @stripe_offset is always small
  enough for u32.

  This saves 8 bytes for the structure.

- map_lookup structure
  This convert @stripe_len to u32, which saves 8 bytes. (saved 4 bytes,
  and removed a 4-bytes hole)
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>