ext4: fix error pointer dereference in ext4_mb_load_buddy_gfp()

This code calls folio_put() on an error pointer which will lead to a
crash.  Check for both error pointers and NULL pointers before calling
folio_put().

Fixes: 5eea586 ("ext4: convert bd_buddy_page to bd_buddy_folio")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/eaafa1d9-a61c-4af4-9f97-d3ad72c60200@moroto.mountain
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -1717,7 +1717,7 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
 	return 0;
 
 err:
-	if (folio)
+	if (!IS_ERR_OR_NULL(folio))
 		folio_put(folio);
 	if (e4b->bd_bitmap_folio)
 		folio_put(e4b->bd_bitmap_folio);