virtiofsd: Add sigreturn to the seccomp whitelist

The virtiofsd currently crashes on s390x. This is because of a
`sigreturn` system call. See audit log below:

type=SECCOMP msg=audit(1669382477.611:459): auid=4294967295 uid=0 gid=0 ses=4294967295 subj=system_u:system_r:virtd_t:s0-s0:c0.c1023 pid=6649 comm="virtiofsd" exe="/usr/libexec/virtiofsd" sig=31 arch=80000016 syscall=119 compat=0 ip=0x3fff15f748a code=0x80000000AUID="unset" UID="root" GID="root" ARCH=s390x SYSCALL=sigreturn

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: German Maglione <gmaglione@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221125143946.27717-1-mhartmay@linux.ibm.com>

tools/virtiofsd/passthrough_seccomp.c

@@ -110,6 +110,7 @@ static const int syscall_allowlist[] = {
 #endif
     SCMP_SYS(set_robust_list),
     SCMP_SYS(setxattr),
+    SCMP_SYS(sigreturn),
     SCMP_SYS(symlinkat),
     SCMP_SYS(syncfs),
     SCMP_SYS(time), /* Rarely needed, except on static builds */