nfsd: add a "GetVersion" upcall for nfsdcld

Add a "GetVersion" upcall to allow nfsd to determine the maximum upcall
version that the nfsdcld userspace daemon supports.  If the daemon
responds with -EOPNOTSUPP, then we know it only supports v1.

Signed-off-by: Scott Mayhew <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>

Author: scottmayhew