NFS: Allow text-based mounts via compat_sys_mount

The compat_sys_mount() system call throws EINVAL for text-based NFSv4
mounts.

The text-based mount interface assumes that any mount option blob that
doesn't set the version field to "1" is a C string (ie not a legacy
mount request).  The compat_sys_mount() call treats blobs that don't
set the version field to "1" as an error.  We just relax the check in
compat_sys_mount() a bit to allow C strings to be passed down to the NFSv4
client.

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