sunrpc: clean up properly in gss_mech_unregister()

hulk inclusion
category: bugfix
bugzilla: NA
CVE: CVE-2020-12656

---------------------------

gss_mech_register() calls svcauth_gss_register_pseudoflavor() for each
flavour, but gss_mech_unregister() does not call auth_domain_put().
This is unbalanced and makes it impossible to reload the module.

Change svcauth_gss_register_pseudoflavor() to return the registered
auth_domain, and save it for later release.

Cc: stable@vger.kernel.org (v2.6.12+)
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206651


Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Wenan Mao <maowenan@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>