btrfs: Return EXDEV for cross file system snapshot

EXDEV seems an appropriate error if an operation fails bacause it
crosses file system boundaries.

Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Kusanagi Kouichi <slash@ac.auone-net.jp>
Signed-off-by: Josef Bacik <jbacik@fb.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 9a90445..a692aad 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1573,7 +1573,7 @@
 		if (src_inode->i_sb != file_inode(file)->i_sb) {
 			btrfs_info(BTRFS_I(src_inode)->root->fs_info,
 				   "Snapshot src from another FS");
-			ret = -EINVAL;
+			ret = -EXDEV;
 		} else if (!inode_owner_or_capable(src_inode)) {
 			/*
 			 * Subvolume creation is not restricted, but snapshots