ima: Update MAX_TEMPLATE_NAME_LEN to fit largest reasonable definition

MAX_TEMPLATE_NAME_LEN is used when restoring measurements carried over from
a kexec. It should be set to the length of a template containing all fields
except for 'd' and 'n', which don't need to be accounted for since they
shouldn't be defined in the same template description as 'd-ng' and 'n-ng'.

That length is greater than the current 15, so update using a sizeof() to
show where the number comes from and also can be visually shown to be
correct. The sizeof() is calculated at compile time.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>