cifs: Use kstrtobool() instead of strtobool()

strtobool() is the same as kstrtobool().
However, the latter is more used within the kernel.

In order to remove strtobool() and slightly simplify kstrtox.h, switch to
the other function name.

While at it, include the corresponding header file (<linux/kstrtox.h>)

Signed-off-by: Christophe JAILLET <[e-mail]>
Reviewed-by: Paulo Alcantara <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/cifs_debug.c

···
 #include <linux/fs.h>
 #include <linux/string.h>
 #include <linux/ctype.h>
+#include <linux/kstrtox.h>
 #include <linux/module.h>
 #include <linux/proc_fs.h>
 #include <linux/uaccess.h>
···
 	rc = get_user(c[0], buffer);
 	if (rc)
 		return rc;
-	if (strtobool(c, &bv) == 0)
+	if (kstrtobool(c, &bv) == 0)
 		cifsFYI = bv;
 	else if ((c[0] > '1') && (c[0] <= '9'))
 		cifsFYI = (int) (c[0] - '0'); /* see cifs_debug.h for meanings */
···
 
 	if (count < 3) {
 		/* single char or single char followed by null */
-		if (strtobool(flags_string, &bv) == 0) {
+		if (kstrtobool(flags_string, &bv) == 0) {
 			global_secflags = bv ? CIFSSEC_MAX : CIFSSEC_DEF;
 			return count;
 		} else if (!isdigit(flags_string[0])) {