apparmor: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
security/apparmor/lib.c:139:23: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: John Johansen <john.johansen@canonical.com>