Disable direct reclaim on zvols

Previously, it was possible for the direct reclaim path to be invoked
when a write to a zvol was made. When a zvol is used as a swap device,
this often causes swap requests to depend on additional swap requests,
which deadlocks. We address this by disabling the direct reclaim path
on zvols.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #342