nfs: fold nfs_page_group_lock_subrequests into nfs_lock_and_join_requests

commit 25edbcac upstream.

Fold nfs_page_group_lock_subrequests into nfs_lock_and_join_requests to
prepare for future changes to this code, and move the helpers to write.c
as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>