NFS: Fix up new minorversion= option

The new minorversion= mount option (commit 3fd5be9e) was merged at
the same time as the recent sloppy parser fixes (commit a5a16bae

),
so minorversion= still uses the old value parsing logic.

If the minorversion= option specifies a bogus value, it should fail
with "bad value" not "bad option."

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