NFS: Refactor NFSv4 text-based mount option validation

Clean up: Refactor the part of nfs4_validate_mount_options() that
handles text-based options, so we can call it from the NFSv2/v3
option validation function.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>