filemap: Handle error return from __filemap_get_folio()

Smatch reports that filemap_fault() was missed in the conversion of
__filemap_get_folio() error returns from NULL to ERR_PTR.

Fixes: 66dabbb6 ("mm: return an ERR_PTR from __filemap_get_folio")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Reported-by: syzbot+48011b86c8ea329af1b9@syzkaller.appspotmail.com
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -3378,7 +3378,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 	 * re-find the vma and come back and find our hopefully still populated
 	 * page.
 	 */
-	if (folio)
+	if (!IS_ERR(folio))
 		folio_put(folio);
 	if (mapping_locked)
 		filemap_invalidate_unlock_shared(mapping);