cifs: add pid field to cifs_writedata

We'll need this to handle rwpidforward option correctly when we use
async writes in the aio_write op.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>

diff --cc fs/cifs/cifsproto.h
Simple merge

diff --cc fs/cifs/cifssmb.c
Simple merge

diff --cc fs/cifs/file.c
Simple merge