Fix zdb -E segfault

SPA_MAXBLOCKSIZE is too large for stack.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #7099

cmd/zdb/zdb.c

@@ -4139,11 +4139,12 @@ zdb_embedded_block(char *thing)
 {
 	blkptr_t bp;
 	unsigned long long *words = (void *)&bp;
-	char buf[SPA_MAXBLOCKSIZE];
+	char *buf;
 	int err;
 
-	memset(&bp, 0, sizeof (blkptr_t));
+	buf = umem_alloc(SPA_MAXBLOCKSIZE, UMEM_NOFAIL);
 
+	bzero(&bp, sizeof (bp));
 	err = sscanf(thing, "%llx:%llx:%llx:%llx:%llx:%llx:%llx:%llx:"
 	    "%llx:%llx:%llx:%llx:%llx:%llx:%llx:%llx",
 	    words + 0, words + 1, words + 2, words + 3,
@@ -4161,6 +4162,7 @@ zdb_embedded_block(char *thing)
 		exit(1);
 	}
 	zdb_dump_block_raw(buf, BPE_GET_LSIZE(&bp), 0);
+	umem_free(buf, SPA_MAXBLOCKSIZE);
 }
 
 int