Illumos #3740

3740 Poor ZFS send / receive performance due to snapshot
     hold / release processing
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:
  https://www.illumos.org/issues/3740
  illumos/illumos-gate@a7a845e4bf

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775

Porting notes:

1. 13fe019870 introduced a merge conflict
   in dsl_dataset_user_release_tmp where some variables were moved
   outside of the preprocessor directive.

2. dea9dfefdd747534b3846845629d2200f0616dad made the previous merge
   conflict worse by switching KM_SLEEP to KM_PUSHPAGE. This is notable
   because this commit refactors the code, adding a new KM_SLEEP
   allocation. It is not clear to me whether this should be converted
   to KM_PUSHPAGE.

3. We had a merge conflict in libzfs_sendrecv.c because of copyright
   notices.

4. Several small C99 compatibility fixed were made.

module/zfs/zfs_ioctl.c

@@ -28,6 +28,7 @@
  * Copyright (c) 2012, Joyent, Inc. All rights reserved.
  * Copyright (c) 201i3 by Delphix. All rights reserved.
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
+ * Copyright (c) 2013 Steven Hartland. All rights reserved.
  */
 
 /*
@@ -4867,20 +4868,6 @@ zfs_ioc_get_holds(const char *snapname, nvlist_t *args, nvlist_t *outnvl)
 static int
 zfs_ioc_release(const char *pool, nvlist_t *holds, nvlist_t *errlist)
 {
-	nvpair_t *pair;
-	int err;
-
-	/*
-	 * The release may cause the snapshot to be destroyed; make sure it
-	 * is not mounted.
-	 */
-	for (pair = nvlist_next_nvpair(holds, NULL); pair != NULL;
-	    pair = nvlist_next_nvpair(holds, pair)) {
-		err = zfs_unmount_snap(nvpair_name(pair));
-		if (err != 0)
-			return (err);
-	}
-
 	return (dsl_dataset_user_release(holds, errlist));
 }