LU-12992 gss: retry in case of short computed shared key

LU-12992 gss: retry in case of short computed shared key

Lustre uses OpenSSL's DH_compute_key() to compute shared secret key.
There is around 1 chance out of 256 that the returned shared key
is shorter than expected.
https://www.qacafe.com/articles/router-vpn-implementation-pitfalls/
If the returned key is short by 1 or 2 bytes, we prepend it with 0s.
Otherwise, we retry twice as it should finally be the expected length.

Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small
Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small
Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small
Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small
Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I519194ef52539ca9a6305120fbf00b60ca985b1f
Reviewed-on: https://review.whamcloud.com/37064
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jeremy Filizetti <jeremy.filizetti@gmail.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>