kexec_file: Disable at runtime if securelevel has been set

kexec_file: Disable at runtime if securelevel has been set

When KEXEC_VERIFY_SIG is not enabled, kernel should not loads image
through kexec_file systemcall if securelevel has been set.

This code was showed in Matthew's patch but not in git:
https://lkml.org/lkml/2015/3/13/778

Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Gbp-Pq: Topic features/all/lockdown
Gbp-Pq: Name 0046-kexec_file-Disable-at-runtime-if-securelevel-has-bee.patch