seccomp sandbox: restrict mremap flags

In the seccomp sandbox, restrict use of mremap(2) to only permit
its use with the MREMAP_MAYMOVE flag and specifically not the
MREMAP_FIXED flag, as the latter may have some use as part of an
attack chain.

Reported by: Mohammad Hossein Abedini <mhmd.abedinii@gmail.com>

ok deraadt@

sandbox-seccomp-filter.c

@@ -367,7 +367,7 @@ static const struct sock_filter preauth_insns[] = {
 	SC_ALLOW_ARG_MASK(__NR_mprotect, 2, PROT_READ|PROT_WRITE|PROT_NONE),
 #endif
 #ifdef __NR_mremap
-	SC_ALLOW(__NR_mremap),
+	SC_ALLOW_ARG_MASK(__NR_mremap, 3, MREMAP_MAYMOVE),
 #endif
 #ifdef __NR_munmap
 	SC_ALLOW(__NR_munmap),