runsc: allow openat for runsc-race

I see that runsc-race is killed by SIGSYS, because openat isn't
allowed by seccomp filters:
60052 openat(AT_FDCWD, "/proc/sys/vm/overcommit_memory",
			O_RDONLY|O_CLOEXEC <unfinished ...>
60052 <... openat resumed> )            = 257
60052 --- SIGSYS {si_signo=SIGSYS, si_code=SYS_SECCOMP, si_call_addr=0xfaacf1,
		si_syscall=__NR_openat, si_arch=AUDIT_ARCH_X86_64} ---

PiperOrigin-RevId: 255640808

runsc/boot/filter/extra_filters_race.go

@@ -33,6 +33,7 @@ func instrumentationFilters() seccomp.SyscallRules {
 		syscall.SYS_MUNLOCK:         {},
 		syscall.SYS_NANOSLEEP:       {},
 		syscall.SYS_OPEN:            {},
+		syscall.SYS_OPENAT:          {},
 		syscall.SYS_SET_ROBUST_LIST: {},
 		// Used within glibc's malloc.
 		syscall.SYS_TIME: {},