Fix resilver writes in vdev_indirect_io_start

This patch addresses an issue found in ztest where resilver
write zios that were passed to an indirect vdev would end up
being handled as though they were resilver read zios. This
caused issues where the zio->io_abd would be both read to
and written from at the same time, causing asserts to fail.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8193