Minor zstream redup command fixes

* Fix uninitialized variable in `zstream redup` command.  The
  'rdt.ddt_count' variable is uninitialized because it was
  allocated from the stack and not globally.  Initialize it.
  This was reported by gcc when compiling with debugging enabled.

    zstream_redup.c:157:16: error: 'rdt.ddt_count' may be used
    uninitialized in this function [-Werror=maybe-uninitialized]

* Remove the cmd/zstreamdump/.gitignore file.  It's no longer
  needed now that the zstreamdump command is a script.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10192

cmd/zstream/zstream_redup.c

@@ -218,6 +218,7 @@ zfs_redup_stream(int infd, int outfd, boolean_t verbose)
 	rdt.ddecache = umem_cache_create("rde", sizeof (redup_entry_t), 0,
 	    NULL, NULL, NULL, NULL, NULL, 0);
 	rdt.numhashbits = highbit64(numbuckets) - 1;
+	rdt.ddt_count = 0;
 
 	char *buf = safe_calloc(bufsz);
 	FILE *ofp = fdopen(infd, "r");