ima: based on policy verify firmware signatures (pre-allocated buffer)

Don't differentiate, for now, between kernel_read_file_id READING_FIRMWARE
and READING_FIRMWARE_PREALLOC_BUFFER enumerations.

Fixes: a098ecd2 firmware: support loading into a pre-allocated buffer (since 4.8)
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>