seccomp: use per arch shmat_syscall

At the beginning of seccomp_memory_deny_write_execute architectures
can set individual filter_syscall, block_syscall, shmat_syscall values.
The former two are then used in the call to add_seccomp_syscall_filter
but shmat_syscall is not.

Right now all shmat_syscall values are the same, so the change is a
no-op, but if ever an architecture is added/modified this would be a
subtle source for a mistake so fix it by using shmat_syscall later.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>

src/shared/seccomp-util.c

@@ -1667,7 +1667,7 @@ int seccomp_memory_deny_write_execute(void) {
 #endif
 
                 if (shmat_syscall > 0) {
-                        r = add_seccomp_syscall_filter(seccomp, arch, SCMP_SYS(shmat),
+                        r = add_seccomp_syscall_filter(seccomp, arch, shmat_syscall,
                                                        1,
                                                        SCMP_A2(SCMP_CMP_MASKED_EQ, SHM_EXEC, SHM_EXEC));
                         if (r < 0)