Btrfs only supports sectorsize 4K, 8K, 16K, 32K, 64K for now, thus for
systems with 4K page size, there is no way the fs is subpage (sectorsize
< PAGE_SIZE).

So here we define btrfs_is_subpage() different according to the
PAGE_SIZE:

- PAGE_SIZE > 4K
  We may hit real subpage cases, define btrfs_is_subpage() as a regular
  function and do the usual checks.

- PAGE_SIZE == 4K (no smaller PAGE_SIZE support AFAIK)
  There is no way the fs is subpage, so just define btrfs_is_subpage()
  as an inline function which always return false.

This saves about 7K bytes for x86_64 debug builds:

	   text	   data	    bss	    dec	    hex	filename
Before:	1484452	 168693	  25776	1678921	 199e49	fs/btrfs/btrfs.ko
After:	1476605	 168445	  25776	1670826	 197eaa	fs/btrfs/btrfs.ko
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>