sunrpc: trim off EC bytes in GSSAPI v2 unwrap

As Bruce points out in RFC 4121, section 4.2.3:

   "In Wrap tokens that provide for confidentiality, the first 16 octets
    of the Wrap token (the "header", as defined in section 4.2.6), SHALL
    be appended to the plaintext data before encryption.  Filler octets
    MAY be inserted between the plaintext data and the "header.""

...and...

   "In Wrap tokens with confidentiality, the EC field SHALL be used to
    encode the number of octets in the filler..."

It's possible for the client to stuff different data in that area on a
retransmission, which could make the checksum come out wrong in the DRC
code.

After decrypting the blob, we should trim off any extra count bytes in
addition to the checksum blob.

Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>