ZIL: Avoid dbuf_read() before dmu_sync().

In most cases dmu_sync() works with dirty records directly and does
not need actual data. The only exception is dmu_sync_late_arrival().
To save some CPU time use dmu_buf_hold_noread*() in z*_get_data()
and explicitly call dbuf_read() in dmu_sync_late_arrival(). There
is also a chance that by that time TXG will already be synced and
we won't have to do it at all.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15153

module/zfs/zfs_vnops.c

@@ -917,8 +917,8 @@ zfs_get_data(void *arg, uint64_t gen, lr_write_t *lr, char *buf,
 		}
 #endif
 		if (error == 0)
-			error = dmu_buf_hold(os, object, offset, zgd, &db,
-			    DMU_READ_NO_PREFETCH);
+			error = dmu_buf_hold_noread(os, object, offset, zgd,
+			    &db);
 
 		if (error == 0) {
 			blkptr_t *bp = &lr->lr_blkptr;