Don't retake already held config lock in zvol_set_volsize

dmu_objset_hold takes the rw_config_lock. So does
dsl_prop_get_integer. On lock taking in a debug build, we assert that
we don't already hold the lock. Calling dsl_prop_get_integer after
dmu_objset_hold triggers this assert.

Fix this by moving to dsl_prop_get_int_ds.
Temporarily use TXG_NOWAIT inside zvol_update_volsize

The caller of zvol_update_volsize holds the dp_config_rwlock, so we
can't use TXG_WAIT. We switch to TXG_NOWAIT as temporary workaround,
however this means that the function could fail, when we can't get the
ZAP tx allocated into a tx group non-blockingly.