Btrfs: check return value of lookup_extent_mapping() correctly

This patch corrects error checking of lookup_extent_mapping().

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>

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

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

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