LU-15508 gss: protect from arbitrary write to init channel

LU-15508 gss: protect from arbitrary write to init channel

In case some arbitrary data was written to the gss init channel,
directly return -EINVAL. This protects against unsolicited
authentication requests, and avoids having a dandling entry
in the auth init cache.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iadde630012e4ded83f9609fbb3e10b2e092deb57
Reviewed-on: https://review.whamcloud.com/46461
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Jeremy Filizetti <jeremy.filizetti@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>