selftests/vm: write strlen length instead of sizeof to nr_hugepages

When setting back the initial value to nr_hugepages, the
test was writing a length sizeof of the string and checking
that strlen was writen. Since those values are not the same,
use strlen in both place instead.

Also make the error messages more explicit to help in future
debugging.

Signed-off-by: Yannick Brosseau <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/vm/compaction_test.c

···
 
 	/* Start with the initial condition of 0 huge pages*/
 	if (write(fd, "0", sizeof(char)) != sizeof(char)) {
-		perror("Failed to write to /proc/sys/vm/nr_hugepages\n");
+		perror("Failed to write 0 to /proc/sys/vm/nr_hugepages\n");
 		goto close_fd;
 	}
 
···
 	/* Request a large number of huge pages. The Kernel will allocate
 	   as much as it can */
 	if (write(fd, "100000", (6*sizeof(char))) != (6*sizeof(char))) {
-		perror("Failed to write to /proc/sys/vm/nr_hugepages\n");
+		perror("Failed to write 100000 to /proc/sys/vm/nr_hugepages\n");
 		goto close_fd;
 	}
 
 	lseek(fd, 0, SEEK_SET);
 
 	if (read(fd, nr_hugepages, sizeof(nr_hugepages)) <= 0) {
-		perror("Failed to read from /proc/sys/vm/nr_hugepages\n");
+		perror("Failed to re-read from /proc/sys/vm/nr_hugepages\n");
 		goto close_fd;
 	}
 
···
 	printf("No of huge pages allocated = %d\n",
 	       (atoi(nr_hugepages)));
 
-	if (write(fd, initial_nr_hugepages, sizeof(initial_nr_hugepages))
+	if (write(fd, initial_nr_hugepages, strlen(initial_nr_hugepages))
 	    != strlen(initial_nr_hugepages)) {
-		perror("Failed to write to /proc/sys/vm/nr_hugepages\n");
+		perror("Failed to write value to /proc/sys/vm/nr_hugepages\n");
 		goto close_fd;
 	}