gss_krb5: document that we ignore sequence number

A couple times recently somebody has noticed that we're ignoring a
sequence number here and wondered whether there's a bug.

In fact, there's not.  Thanks to Andy Adamson for pointing out a useful
explanation in rfc 2203.  Add comments citing that rfc, and remove
"seqnum" to prevent static checkers complaining about unused variables.

Reported-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --cc net/sunrpc/auth_gss/gss_krb5_unseal.c
Simple merge

diff --cc net/sunrpc/auth_gss/gss_krb5_wrap.c
Simple merge