Improve write performance by using dmu_read_by_dnode()

In zfs_log_write(), we can use dmu_read_by_dnode() rather than
dmu_read() thus avoiding unnecessary dnode_hold() calls.

We get a 2-5% performance gain for large sequential_writes tests, >=128K
writes to files with recordsize=8K.

Testing done on Ubuntu 18.04 with 4.15 kernel, 8vCPUs and SSD storage on
VMware ESX.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Nguyen <tony.nguyen@delphix.com>
Closes #9156

module/zfs/zfs_log.c

@@ -42,6 +42,7 @@
 #include <sys/mode.h>
 #include <sys/acl.h>
 #include <sys/dmu.h>
+#include <sys/dbuf.h>
 #include <sys/spa.h>
 #include <sys/zfs_fuid.h>
 #include <sys/dsl_dataset.h>
@@ -510,6 +511,7 @@ zfs_log_write(zilog_t *zilog, dmu_tx_t *tx, int txtype,
     znode_t *zp, offset_t off, ssize_t resid, int ioflag,
     zil_callback_t callback, void *callback_data)
 {
+	dmu_buf_impl_t *db = (dmu_buf_impl_t *)sa_get_db(zp->z_sa_hdl);
 	uint32_t blocksize = zp->z_blksz;
 	itx_wr_state_t write_state;
 	uintptr_t fsync_cnt;
@@ -556,13 +558,16 @@ zfs_log_write(zilog_t *zilog, dmu_tx_t *tx, int txtype,
 		itx = zil_itx_create(txtype, sizeof (*lr) +
 		    (wr_state == WR_COPIED ? len : 0));
 		lr = (lr_write_t *)&itx->itx_lr;
-		if (wr_state == WR_COPIED && dmu_read(ZTOZSB(zp)->z_os,
-		    zp->z_id, off, len, lr + 1, DMU_READ_NO_PREFETCH) != 0) {
+
+		DB_DNODE_ENTER(db);
+		if (wr_state == WR_COPIED && dmu_read_by_dnode(DB_DNODE(db),
+		    off, len, lr + 1, DMU_READ_NO_PREFETCH) != 0) {
 			zil_itx_destroy(itx);
 			itx = zil_itx_create(txtype, sizeof (*lr));
 			lr = (lr_write_t *)&itx->itx_lr;
 			wr_state = WR_NEED_COPY;
 		}
+		DB_DNODE_EXIT(db);
 
 		itx->itx_wr_state = wr_state;
 		lr->lr_foid = zp->z_id;