pNFS: Use the standard I/O stateid when calling LAYOUTGET

Instead of having a private method for copying the open/delegation stateid,
use the same call that is used for standard I/O through the MDS.

Note that this means we transmit the stateid with a zero seqid, avoiding
issues with NFS4ERR_OLD_STATEID.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>