ima: add gid support

IMA currently supports the concept of rules based on uid where the rule
is based on the uid of the file owner or the uid of the user accessing
the file. Provide the ability to have similar rules based on gid.

Signed-off-by: Curtis Veit <veit@vpieng.com>
Co-developed-by: Alex Henrie <alexh@vpitech.com>
Signed-off-by: Alex Henrie <alexh@vpitech.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>