NFSD: MKNOD should return NFSERR_BADTYPE instead of NFSERR_INVAL

A late paragraph of RFC 1813 Section 3.3.11 states:

| ... if the server does not support the target type or the
| target type is illegal, the error, NFS3ERR_BADTYPE, should
| be returned. Note that NF3REG, NF3DIR, and NF3LNK are
| illegal types for MKNOD.

The Linux NFS server incorrectly returns NFSERR_INVAL in these
cases.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>