kernfs: checking for IS_ERR() instead of NULL

The kernfs_get_inode() returns NULL on error, it never returns error
pointers.

Fixes: aa8188253474 ("kernfs: add exportfs operations")
Acked-by: Tejun Heo <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

fs/kernfs/mount.c

···
 		return ERR_PTR(-ESTALE);
 	inode = kernfs_get_inode(sb, kn);
 	kernfs_put(kn);
-	if (IS_ERR(inode))
-		return ERR_CAST(inode);
+	if (!inode)
+		return ERR_PTR(-ESTALE);
 
 	if (generation && inode->i_generation != generation) {
 		/* we didn't find the right inode.. */