Btrfs: return error of btrfs_update_inode() to caller

We didn't check error of btrfs_update_inode(), but that error looks
easy to bubble back up.

Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>