gfs2: Fix a NULL vs IS_ERR() bug in gfs2_find_jhead()

The filemap_grab_folio() function doesn't return NULL, it returns error
pointers.  Fix the check to match.

Fixes: 4082976009 ("gfs2: Convert gfs2_find_jhead() to use a folio")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/lops.c

@@ -530,8 +530,8 @@ int gfs2_find_jhead(struct gfs2_jdesc *jd, struct gfs2_log_header_host *head,
 			if (!folio) {
 				folio = filemap_grab_folio(mapping,
 						block >> shift);
-				if (!folio) {
-					ret = -ENOMEM;
+				if (IS_ERR(folio)) {
+					ret = PTR_ERR(folio);
 					done = true;
 					goto out;
 				}