Fix stack zio_done()

Fix stack zio_done()

Eliminated local variables pointing to members of the zio struct.
Just refer to the struct members directly.  This saved about 32 bytes per
call, but this function can be called recurisvely up to 19 levels deep,
so we potentially save up to 608 bytes.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>