As 'cr_gid' was in fact stored in cr_groups[0],
rule_grant_supplementary_groups() would loop only on further elements of
cr_groups[]. Now that cr_groups[0] is not 'cr_gid' anymore, but some
supplementary group, take it into account.
cred: Update some comments after 'cr_gid' is no more cr_groups[0]
Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
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