nfscl: Check for a forced dismount in nfscl_getref()

The nfscl_getref() function is called within nfscl_doiods() when
the NFSv4.1/4.2 pNFS client is doing I/O on a DS. As such,
nfscl_getref() needs to check for a forced dismount.
This patch adds that check.

Found during a recent IETF NFSv4 working group testing event.

MFC after: 2 weeks