selftests: cachestat: use proper syscall number macro

At the moment the cachestat syscall number is hard coded into the test
source code.
Remove that and replace it with the proper __NR_cachestat macro.
That ensures compatibility should other architectures pick a different
number.

Signed-off-by: Andre Przywara <[e-mail]>
Reviewed-by: Nhat Pham <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/cachestat/test_cachestat.c

···
 	"/dev/zero", "/dev/null", "/dev/urandom",
 	"/proc/version", "/proc"
 };
-static const int cachestat_nr = 451;
 
 void print_cachestat(struct cachestat *cs)
 {
···
 		}
 	}
 
-	syscall_ret = syscall(cachestat_nr, fd, &cs_range, &cs, 0);
+	syscall_ret = syscall(__NR_cachestat, fd, &cs_range, &cs, 0);
 
 	ksft_print_msg("Cachestat call returned %ld\n", syscall_ret);
 
···
 			ksft_print_msg("fsync fails.\n");
 			ret = false;
 		} else {
-			syscall_ret = syscall(cachestat_nr, fd, &cs_range, &cs, 0);
+			syscall_ret = syscall(__NR_cachestat, fd, &cs_range, &cs, 0);
 
 			ksft_print_msg("Cachestat call (after fsync) returned %ld\n",
 				syscall_ret);
···
 		goto close_fd;
 	}
 
-	syscall_ret = syscall(cachestat_nr, fd, &cs_range, &cs, 0);
+	syscall_ret = syscall(__NR_cachestat, fd, &cs_range, &cs, 0);
 
 	if (syscall_ret) {
 		ksft_print_msg("Cachestat returned non-zero.\n");