apparmor: store return value of unpack_perms_table() to signed variable

The unpack_perms_table() can return error which is negative value. Store
the return value to a signed variable. policy->size is unsigned
variable. It shouldn't be used to store the return status.

Fixes: 2d6b2dea7f3c ("apparmor: add the ability for policy to specify a permission table")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>