netfilter: xt_owner: Add supplementary groups option

netfilter: xt_owner: Add supplementary groups option

The XT_OWNER_SUPPL_GROUPS flag causes GIDs specified with XT_OWNER_GID
to be also checked in the supplementary groups of a process.

f_cred->group_info cannot be modified during its lifetime and f_cred
holds a reference to it so it's safe to use.

Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@samsung.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
backport from mainline to apply supplementary groups on netfilter

Change-Id: I900d03042c82b4d5f13dec7695e4b1eba7f2aa74
(cherry picked from commit e7a26f2b0e8021c9415f0b92caf9538974720855)