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>