Increase default zcmd allocation to 256K (LP: #567557)

When creating hundreds of clones (for example using containers with
LXD) cloning slows down as the number of clones increases over time.
The reason for this is that the fetching of the clone information
using a small zcmd buffer requires two ioctl calls, one to determine
the size and a second to return the data. However, this requires
gathering the data twice, once to determine the size and again to
populate the zcmd buffer to return it to userspace.

These are expensive ioctl() calls, so instead, make the default buffer
size much larger: 256K. This may sound large, but on 64 bit systems
running ZFS this is not a huge chunk of memory for the speed
improvement we gains for large sets of clones:

        16K zcmd        256K zcmd
Clones	Time    Clones  Time    Clone   % improvement
        (secs) 	per sec (secs)  per sec
100	7	14.29	5	20.00	28.57
200	10	20.00	9	22.22	10.00
300	19	15.79	18	16.67	5.26
400	22	18.18	22	18.18	0.00
500	29	17.24	29	17.24	0.00
600	39	15.38	39	15.38	0.00
700	46	15.22	45	15.56	2.17
800	58	13.79	51	15.69	12.07
900	74	12.16	61	14.75	17.57
1000	90	11.11	74	13.51	17.78
1100	98	11.22	87	12.64	11.22
1200	102	11.76	95	12.63	6.86
1300	113	11.50	104	12.50	7.96
1400	143	9.79	109	12.84	23.78
1500	145	10.34	132	11.36	8.97
1600	165	9.70	145	11.03	12.12
1700	187	9.09	156	10.90	16.58
1800	210	8.57	166	10.84	20.95
1900	226	8.41	183	10.38	19.03
2000	256	7.81	198	10.10	22.66
2200	311	7.07	238	9.24	23.47
2400	373	6.43	271	8.86	27.35
2600	487	5.34	316	8.23	35.11
3000	619	4.85	426	7.04	31.18
3400	915	3.72	549	6.19	40.00
4000	1332	3.00	923	4.33	30.71

As one can see, with > 2000 clones we get 25-40% speed
improvement.

This patch was originally suggested by Brian Behlendorf
(see https://github.com/zfsonlinux/zfs/issues/6372), however
this fix is a more generic fix to cover all zcmd cases.

Signed-off-by: Colin Ian King <colin.king@canonical.com>

Index: zfs/lib/libzfs/libzfs_util.c
===================================================================
--- zfs.orig/lib/libzfs/libzfs_util.c
+++ zfs/lib/libzfs/libzfs_util.c
@@ -1354,7 +1354,7 @@ int
 zcmd_alloc_dst_nvlist(libzfs_handle_t *hdl, zfs_cmd_t *zc, size_t len)
 {
 	if (len == 0)
-		len = 16 * 1024;
+		len = 256 * 1024;
 	zc->zc_nvlist_dst_size = len;
 	zc->zc_nvlist_dst =
 	    (uint64_t)(uintptr_t)zfs_alloc(hdl, zc->zc_nvlist_dst_size);