NFSv4: Send implementation id with exchange_id

Send the nfs implementation id in EXCHANGE_ID requests unless the module
parameter nfs.send_implementation_id is 0.

This adds a CONFIG variable for the nii_domain that defaults to "kernel.org".

Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>