Stop wasting time on malloc in snprintf_zstd_header

Profiling zdb -vvvvv on datasets with a lot of zstd blocks, we find
ourselves spending quite a lot of time on malloc/free, because we
allocate a 16M abd each call, and never free it, so we're leaking
16M per call as well.

This seems sub-optimal. So let's just keep the buffer around and
reuse it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#15721

cmd/zdb/zdb.c

@@ -2361,7 +2361,7 @@ static void
 snprintf_zstd_header(spa_t *spa, char *blkbuf, size_t buflen,
     const blkptr_t *bp)
 {
-	abd_t *pabd;
+	static abd_t *pabd = NULL;
 	void *buf;
 	zio_t *zio;
 	zfs_zstdhdr_t zstd_hdr;
@@ -2392,7 +2392,8 @@ snprintf_zstd_header(spa_t *spa, char *blkbuf, size_t buflen,
 		return;
 	}
 
-	pabd = abd_alloc_for_io(SPA_MAXBLOCKSIZE, B_FALSE);
+	if (!pabd)
+		pabd = abd_alloc_for_io(SPA_MAXBLOCKSIZE, B_FALSE);
 	zio = zio_root(spa, NULL, NULL, 0);
 
 	/* Decrypt but don't decompress so we can read the compression header */