zfs_log_write: simplify data copying code for WR_COPIED records

lr_write_t records that are WR_COPIED have the record data directly
appended to them (see lr_write_t type definition).

The data is copied from the debuf using dmu_read_by_dnode.

This function was called, only for WR_COPIED records, as part of a
short-circuiting if-statement's if-expression.

I found this side-effectful call to dmu_read_by_dnode pretty
hard to spot.
This patch improves readability by moving the call to its own line.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #10956

module/zfs/zfs_log.c

@@ -584,15 +584,22 @@ zfs_log_write(zilog_t *zilog, dmu_tx_t *tx, int txtype,
 		    (wr_state == WR_COPIED ? len : 0));
 		lr = (lr_write_t *)&itx->itx_lr;
 
-		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;
+		/*
+		 * For WR_COPIED records, copy the data into the lr_write_t.
+		 */
+		if (wr_state == WR_COPIED) {
+			int err;
+			DB_DNODE_ENTER(db);
+			err = dmu_read_by_dnode(DB_DNODE(db), off, len, lr + 1,
+			    DMU_READ_NO_PREFETCH);
+			if (err != 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);
 		}
-		DB_DNODE_EXIT(db);
 
 		itx->itx_wr_state = wr_state;
 		lr->lr_foid = zp->z_id;