Documentation: make configfs example code simpler, clearer

If "p" is NULL then it will cause an oops when we pass it to
simple_strtoul().  In this case "p" can not be NULL so I removed the
check.  I also changed the check a little to make it more explicit that
we are testing whether p points to the NUL char.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/configfs/configfs_example_explicit.c

@@ -89,7 +89,7 @@ static ssize_t childless_storeme_write(struct childless *childless,
 	char *p = (char *) page;
 
 	tmp = simple_strtoul(p, &p, 10);
-	if (!p || (*p && (*p != '\n')))
+	if ((*p != '\0') && (*p != '\n'))
 		return -EINVAL;
 
 	if (tmp > INT_MAX)