memblock tests: add command line help option

Add a help command line option to the help message. Add the help option
to the short and long options so it will be recognized as a valid
option.

Usage:
    $ ./main -h

    Or:
    $ ./main --help
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Rebecca Mckeever <remckee0@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/0f3b93a79de78c0da1ca90f74fe35e9a85c7cf93.1660451025.git.remckee0@gmail.com

tools/testing/memblock/tests/common.c

@@ -14,14 +14,16 @@ static struct test_memory memory_block;
 static const char __maybe_unused *prefixes[PREFIXES_MAX];
 static int __maybe_unused nr_prefixes;
 
-static const char *short_opts = "mv";
+static const char *short_opts = "hmv";
 static const struct option long_opts[] = {
+	{"help", 0, NULL, 'h'},
 	{"movable-node", 0, NULL, 'm'},
 	{"verbose", 0, NULL, 'v'},
 	{NULL, 0, NULL, 0}
 };
 
 static const char * const help_opts[] = {
+	"display this help message and exit",
 	"disallow allocations from regions marked as hotplugged\n\t\t\t"
 		"by simulating enabling the \"movable_node\" kernel\n\t\t\t"
 		"parameter",