ima: fix mprotect checking

ima: fix mprotect checking

Make sure IMA is enabled before checking mprotect change.  Addresses
report of a 3.7% regression of boot-time.dhcp.

Fixes: ed505b2b2e17 ("ima: verify mprotect change is consistent with mmap policy")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Reviewed-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Tested-by: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>