ima: add a knob ima= to allow disabling IMA in kdump kernel

ima: add a knob ima= to allow disabling IMA in kdump kernel

Kdump kernel doesn't need IMA functionality, and enabling IMA will cost
extra memory. It would be very helpful to allow IMA to be disabled for
kdump kernel.

Hence add a knob ima=on|off here to allow turning IMA off in kdump
kernel if needed.

Note that this IMA disabling is limited to kdump kernel, please don't
abuse it in other kernel and thus serious consequences are caused.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>