Btrfs: lock_and_cleanup_extent_if_need: Work with extents aligned to blocksize.

This is a small part of a bigger set of fixes needed to get
subpagesize-blocksize to function properly. This patch is required to get 2k
blocksize writes to work correctly.

Without this patch, two consecutive 2k block writes that map to first and
second 2k parts of a 4k page would result in the second 2k write to *clear*
out the EXTENT_DELALLOC bit of the first 2k block. Fix this.

Signed-off-by: chandan <chandan@linux.vnet.ibm.com>