Call dmu_read_uio_dbuf() in zvol_read()

The difference between `dmu_read_uio()` and `dmu_read_uio_dbuf()` is
that the former takes a hold while the latter uses an existing hold.
`zfs_read()` in the ZPL will use `dmu_read_uio_dbuf()` while
our analogous `zvol_write()` will use `dmu_write_uio_dbuf()`, but for no
apparent reason, we inherited a `zvol_read()` function from
OpenSolaris that does `dmu_read_uio()`. illumos-gate also still
uses `dmu_read_uio()` to this day. Lets switch to `dmu_read_uio_dbuf()`,
which is more performant.

Signed-off-by: Richard Yao <ryao@gentoo.org>

module/zfs/zvol.c

@@ -769,7 +769,7 @@ zvol_read(struct bio *bio)
 		if (bytes > volsize - uio.uio_loffset)
 			bytes = volsize - uio.uio_loffset;
 
-		error =  dmu_read_uio(zv->zv_objset, ZVOL_OBJ, &uio, bytes);
+		error = dmu_read_uio_dbuf(zv->zv_dbuf, &uio, bytes);
 		if (error) {
 			/* convert checksum errors into IO errors */
 			if (error == ECKSUM)