Btrfs: walk compressed pages based on the nr_pages count instead of bytes

The byte walk counting was awkward and error prone.  This uses the
number of pages sent the higher layer to build bios.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --cc fs/btrfs/compression.c
Simple merge

diff --cc fs/btrfs/inode.c
Simple merge