sysctl: fix int -> unsigned long assignments in INT_MIN case

The following

if (val < 0)
*lvalp = (unsigned long)-val;

is incorrect because the compiler is free to assume -val to be positive
and use a sign-extend instruction for extending the bit pattern. This is
a problem if val == INT_MIN:

# echo -2147483648 >/proc/sys/dev/scsi/logging_level
# cat /proc/sys/dev/scsi/logging_level
-18446744071562067968

Cast to unsigned long before negation - that way we first sign-extend and
then negate an unsigned, which is well defined. With this:

# cat /proc/sys/dev/scsi/logging_level
-2147483648

Signed-off-by: Ilya Dryomov <[e-mail]>
Cc: Mikulas Patocka <[e-mail]>
Cc: Robert Xiao <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Kees Cook <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/sysctl.c

···
 		int val = *valp;
 		if (val < 0) {
 			*negp = true;
-			*lvalp = (unsigned long)-val;
+			*lvalp = -(unsigned long)val;
 		} else {
 			*negp = false;
 			*lvalp = (unsigned long)val;
···
 		int val = *valp;
 		if (val < 0) {
 			*negp = true;
-			*lvalp = (unsigned long)-val;
+			*lvalp = -(unsigned long)val;
 		} else {
 			*negp = false;
 			*lvalp = (unsigned long)val;
···
 		unsigned long lval;
 		if (val < 0) {
 			*negp = true;
-			lval = (unsigned long)-val;
+			lval = -(unsigned long)val;
 		} else {
 			*negp = false;
 			lval = (unsigned long)val;
···
 		unsigned long lval;
 		if (val < 0) {
 			*negp = true;
-			lval = (unsigned long)-val;
+			lval = -(unsigned long)val;
 		} else {
 			*negp = false;
 			lval = (unsigned long)val;
···
 		unsigned long lval;
 		if (val < 0) {
 			*negp = true;
-			lval = (unsigned long)-val;
+			lval = -(unsigned long)val;
 		} else {
 			*negp = false;
 			lval = (unsigned long)val;