NFSv4.1: fix SP4_MACH_CRED protection for pnfs IO

If the client is doing pnfs IO and Kerberos is configured and EXCHANGEID
successfully negotiated SP4_MACH_CRED and WRITE/COMMIT are on the
list of state protected operations, then we need to make sure to
choose the DS's rpc_client structure instead of the MDS's one.

Fixes: fb91fb0e ("NFS: Move call to nfs4_state_protect_write() to nfs4_write_setup()")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>