Add 'null' verify option

For testing purposes.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/options.c b/options.c
index bbb3099..5401f90 100644
--- a/options.c
+++ b/options.c
@@ -531,6 +531,11 @@
 			    .oval = VERIFY_MD5,
 			    .help = "Use md5 checksums for verification",
 			  },
+			  {
+			    .ival = "null",
+			    .oval = VERIFY_NULL,
+			    .help = "Pretend to verify",
+			  },
 		},
 	},
 	{