ZIL: Reduce scope of per-dataset zl_issuer_lock.

Before this change ZIL copied all log data while holding the lock.
It caused huge lock contention on workloads with many big parallel
writes.  This change splits the process into two parts: first,
zil_lwb_assign() estimates the log space needed for all transactions,
and zil_lwb_write_close() allocates blocks and zios while holding the
lock, then, after the lock in dropped, zil_lwb_commit() copies the
data, and zil_lwb_write_issue() issues the I/Os.

Signed-off-by:  Alexander Motin <mav@FreeBSD.org>
Sponsored by:   iXsystems, Inc.

include/sys/zil_impl.h

@@ -44,7 +44,7 @@ extern "C" {
  * must be held.
  *
  * After the lwb is "opened", it can transition into the "issued" state
- * via zil_lwb_write_issue(). Again, the zilog's "zl_issuer_lock" must
+ * via zil_lwb_write_close(). Again, the zilog's "zl_issuer_lock" must
  * be held when making this transition.
  *
  * After the lwb's write zio completes, it transitions into the "write
@@ -94,6 +94,7 @@ typedef struct lwb {
 	boolean_t	lwb_fastwrite;	/* is blk marked for fastwrite? */
 	boolean_t	lwb_slog;	/* lwb_blk is on SLOG device */
 	int		lwb_nused;	/* # used bytes in buffer */
+	int		lwb_nfilled;	/* # filled bytes in buffer */
 	int		lwb_sz;		/* size of block and buffer */
 	lwb_state_t	lwb_state;	/* the state of this lwb */
 	char		*lwb_buf;	/* log write buffer */
@@ -107,6 +108,7 @@ typedef struct lwb {
 	avl_tree_t	lwb_vdev_tree;	/* vdevs to flush after lwb write */
 	kmutex_t	lwb_vdev_lock;	/* protects lwb_vdev_tree */
 	hrtime_t	lwb_issued_timestamp; /* when was the lwb issued? */
+	list_node_t	lwb_issue_node;	/* linkage of lwbs ready for issue */
 } lwb_t;
 
 /*