Illumos 5056 - ZFS deadlock on db_mtx and dn_holds

5056 ZFS deadlock on db_mtx and dn_holds
Author: Justin Gibbs <justing@spectralogic.com>
Reviewed by: Will Andrews <willa@spectralogic.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/5056
  https://github.com/illumos/illumos-gate/commit/bc9014e

Porting Notes:

sa_handle_get_from_db():
  - the original patch includes an otherwise unmentioned fix for a
    possible usage of an uninitialised variable

dmu_objset_open_impl():
  - Under Illumos list_link_init() is the same as filling a list_node_t
    with NULLs, so they don't notice if they miss doing list_link_init()
    on a zero'd containing structure (e.g. allocated with kmem_zalloc as
    here). Under Linux, not so much: an uninitialised list_node_t goes
    "Boom!" some time later when it's used or destroyed.

dmu_objset_evict_dbufs():
  - reduce stack usage using kmem_alloc()

Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/sa_impl.h

@@ -21,6 +21,7 @@
 /*
  * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2013 by Delphix. All rights reserved.
+ * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
  */
 
 #ifndef	_SYS_SA_IMPL_H
@@ -208,11 +209,12 @@ typedef enum sa_data_op {
  */
 
 struct sa_handle {
+	dmu_buf_user_t	sa_dbu;
 	kmutex_t	sa_lock;
 	dmu_buf_t	*sa_bonus;
 	dmu_buf_t	*sa_spill;
 	objset_t	*sa_os;
-	void 		*sa_userp;
+	void		*sa_userp;
 	sa_idx_tab_t	*sa_bonus_tab;	 /* idx of bonus */
 	sa_idx_tab_t	*sa_spill_tab; /* only present if spill activated */
 };