erofs: fix handling kern_mount() failure

if you have a variable that holds NULL or  a pointer to live struct mount,
do not shove ERR_PTR() into it - not if you later treat "not NULL" as
"holds a pointer to object".
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/erofs/fscache.c

@@ -381,11 +381,12 @@ static int erofs_fscache_init_domain(struct super_block *sb)
 		goto out;
 
 	if (!erofs_pseudo_mnt) {
-		erofs_pseudo_mnt = kern_mount(&erofs_fs_type);
-		if (IS_ERR(erofs_pseudo_mnt)) {
-			err = PTR_ERR(erofs_pseudo_mnt);
+		struct vfsmount *mnt = kern_mount(&erofs_fs_type);
+		if (IS_ERR(mnt)) {
+			err = PTR_ERR(mnt);
 			goto out;
 		}
+		erofs_pseudo_mnt = mnt;
 	}
 
 	domain->volume = sbi->volume;