LU-14538 gss: make namespace optional in lgss_keyring

LU-14538 gss: make namespace optional in lgss_keyring

Introduce a new tunable 'sptlrpc.gss.gss_check_upcall_ns' to
make namespace support optional in lgss_keyring.
By default it is set to 1, which means adopt the standard behavior,
consisting in checking caller's namespace and switching namespace
if necessary.
When the tunable is set to 0, lgss_keyring sticks to the current
namespace.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ib9d4e47935a718d4aae31fbb0d13f6bc8a4005a5
Reviewed-on: https://review.whamcloud.com/42112
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>