nfs: fix mis-merged __filemap_get_folio() error check

Fix another case of an incorrect check for the returned 'folio' value
from __filemap_get_folio().

The failure case used to return NULL, but was changed by commit
66dabbb6 ("mm: return an ERR_PTR from __filemap_get_folio").

But in the meantime, commit ec108d3c ("NFS: Convert readdir page
array functions to use a folio") added a new user of that function.

And my merge of the two did not fix this up correctly.

The ext4 merge had the same issue, but that one had been caught in
linux-next and got properly fixed while merging.

Fixes: 0127f25b ("Merge tag 'nfs-for-6.4-1' of git://git.linux-nfs.org/projects/anna/linux-nfs"

)
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>