selinux: stop returning node from avc_insert()

The callers haven't used the returned node since
commit 21193dcd ("SELinux: more careful use of avd in
avc_has_perm_noaudit") and the return value assignments were removed in
commit 0a9876f3 ("selinux: Remove redundant assignments"). Stop
returning the node altogether and make the functions return void.

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
PM: minor subj tweak, repair whitespace damage
Signed-off-by: Paul Moore <paul@paul-moore.com>