fix an architectural problem introduced in r320156, ZFS ABD import

The implementation of ZFS refcount_t uses the emulated illumos mutex
(the sx lock) and the waiting memory allocation when ZFS_DEBUG is
enabled. This makes refcount_t unsuitable for use in GEOM g_up
thread where sleeping is prohibited.

When importing the ABD change I modified vdev_geom using illumos
vdev_disk as an example. As a result, I added a call to abd_return_buf
in vdev_geom_io_intr. The latter is called on g_up thread while the
former uses refcount_t.

This change fixes the problem by deferring the abd_return_buf call to
the previously unused vdev_geom_io_done that is called on a ZFS zio
taskqueue thread where sleeping is allowed.

A side bonus of this change is that now a vdev zio has a pointer
to its corresponding bio while the zio is active.

Reported by: Shawn Webb <shawn.webb@hardenedbsd.org>
Tested by: Shawn Webb <shawn.webb@hardenedbsd.org>
MFC after: 1 week
X-MFC with: r320156