ima: use match_string() helper

match_string() returns the index of an array for a matching string,
which can be used intead of open coded variant.

Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>