LU-13498 gss: update sequence in case of target disconnect

LU-13498 gss: update sequence in case of target disconnect

Client to OST connections can go idle, leading to target disconnect.
In this event, maintaining correct sequence number ensures that GSS
does not erroneously consider requests as replays.
Sequence is normally updated on export destroy, but this can occur too
late, ie after a new target connect request has been processed. So
explicitly update sec context at disconnect time.

Lustre-change: https://review.whamcloud.com/40122
Lustre-commit: 1275857c178fdf6e301345c7588499451c8ffd37

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I65c27e1ab459b2a29670580121ef6e1a00f18918
Reviewed-on: https://review.whamcloud.com/40993
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>