Call dmu_read_uio_dbuf() in zvol_read()

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>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Closes #4316