nfs: move nfs_wait_on_request to write.c

nfs_wait_on_request is now only used in write.c.  Move it there
and mark it static.

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