Fix zsb->z_hold_mtx deadlock

The zfs_znode_hold_enter() / zfs_znode_hold_exit() functions are used to
serialize access to a znode and its SA buffer while the object is being
created or destroyed.  This kind of locking would normally reside in the
znode itself but in this case that's impossible because the znode and SA
buffer may not yet exist.  Therefore the locking is handled externally
with an array of mutexs and AVLs trees which contain per-object locks.

In zfs_znode_hold_enter() a per-object lock is created as needed, inserted
in to the correct AVL tree and finally the per-object lock is held.  In
zfs_znode_hold_exit() the process is reversed.  The per-object lock is
released, removed from the AVL tree and destroyed if there are no waiters.

This scheme has two important properties:

1) No memory allocations are performed while holding one of the z_hold_locks.
   This ensures evict(), which can be called from direct memory reclaim, will
   never block waiting on a z_hold_locks which just happens to have hashed
   to the same index.

2) All locks used to serialize access to an object are per-object and never
   shared.  This minimizes lock contention without creating a large number
   of dedicated locks.

On the downside it does require znode_lock_t structures to be frequently
allocated and freed.  However, because these are backed by a kmem cache
and very short lived this cost is minimal.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4106

include/sys/zfs_vfsops.h

@@ -112,8 +112,9 @@ typedef struct zfs_sb {
 	uint64_t	z_groupquota_obj;
 	uint64_t	z_replay_eof;	/* New end of file - replay only */
 	sa_attr_type_t	*z_attr_table;	/* SA attr mapping->id */
-	uint64_t	z_hold_mtx_size; /* znode hold locks size */
-	kmutex_t	*z_hold_mtx;	/* znode hold locks */
+	uint64_t	z_hold_size;	/* znode hold array size */
+	avl_tree_t	*z_hold_trees;	/* znode hold trees */
+	kmutex_t	*z_hold_locks;	/* znode hold locks */
 } zfs_sb_t;
 
 #define	ZFS_SUPER_MAGIC	0x2fc12fc1