dm linear: eliminate linear_end_io call if CONFIG_DM_ZONED disabled

It is best to avoid any extra overhead associated with bio completion.
DM core will indirectly call a DM target's .end_io if it is defined.
In the case of DM linear, there is no need to do so (for every bio that
completes) if CONFIG_DM_ZONED is not enabled.

Avoiding an extra indirect call for every bio completion is very
important for ensuring DM linear doesn't incur more overhead that
further widens the performance gap between dm-linear and raw block
devices.

Fixes: 0be12c1c ("dm linear: add support for zoned block devices")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>