SELinux: replace weak GFP_ATOMIC to GFP_KERNEL in avc_add_callback

avc_add_callback now only called from initcalls, so replace the
weak GFP_ATOMIC to GFP_KERNEL, and mark this function __init
to make a warning when not been called from initcalls.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Eric Paris <eparis@redhat.com>