samples/bpf: Add missing option to xdpsock usage

Commit 743e568c (samples/bpf: Add a "force" flag to XDP samples)
introduced the '-F' option but missed adding it to the usage() and the
'long_option' array.

Fixes: 743e568c (samples/bpf: Add a "force" flag to XDP samples)
Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191114162847.221770-2-andre.guedes@intel.com

samples/bpf/xdpsock_user.c

@@ -374,6 +374,7 @@ static struct option long_options[] = {
 	{"no-need-wakeup", no_argument, 0, 'm'},
 	{"unaligned", no_argument, 0, 'u'},
 	{"shared-umem", no_argument, 0, 'M'},
+	{"force", no_argument, 0, 'F'},
 	{0, 0, 0, 0}
 };
 
@@ -397,6 +398,7 @@ static void usage(const char *prog)
 		"  -f, --frame-size=n   Set the frame size (must be a power of two in aligned mode, default is %d).\n"
 		"  -u, --unaligned	Enable unaligned chunk placement\n"
 		"  -M, --shared-umem	Enable XDP_SHARED_UMEM\n"
+		"  -F, --force		Force loading the XDP prog\n"
 		"\n";
 	fprintf(stderr, str, prog, XSK_UMEM__DEFAULT_FRAME_SIZE);
 	exit(EXIT_FAILURE);