nfs: fold nfs_folio_find_and_lock_request into nfs_lock_and_join_requests

Fold nfs_folio_find_and_lock_request into the only caller to prepare
for changes to this code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>