mount.nfs: Support "-t nfs,vers=4" mounts in the kernel

    Support "vers=4" in nfs_nfs_version()

    Skip UMNT call for "-t nfs -o vers=4" mounts

	For "-t nfs -o vers=4" mounts, we want to skip v2/v3
	version/transport negotiation, but be sure to append
	the "clientaddr" option.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>
Tested-by: Steve Dickson <steved@redhat.com>