Btrfs: fix use of uninitialized err variable

fs/btrfs/file.c: In function ‘prepare_pages.isra.18’:
fs/btrfs/file.c:1265:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>