nfs: fix another case of NULL/IS_ERR confusion wrt folio pointers

Dan has been improving on the smatch error pointer checks, and pointed
at another case where the __filemap_get_folio() conversion to error
pointers had been overlooked.  This time because it was hidden behind
the filemap_grab_folio() helper function that is a wrapper around it.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>