netfs: Fix a NULL vs IS_ERR() check in netfs_perform_write()

The netfs_grab_folio_for_write() function doesn't return NULL, it returns
error pointers.  Update the check accordingly.

Fixes: c38f4e96 ("netfs: Provide func to copy data to pagecache for buffered write")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/29fb1310-8e2d-47ba-b68d-40354eb7b896@moroto.mountain/