poll: generate POLLERR even if no events requested

According to Posix (and to actual Linux behavior, as verified by another
test we add to tst-pipe.cc), the POLLERR event should be generated even
if the user didn't ask for it in poll() - and even if the user didn't ask
for *any* event (events==0).

As far as I can tell, this fixes the last known poll() incompatibility
with Linux, as described in issue #35. So fixes #35.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Revert "vfs/zfs: remove per-file lock enforcement for ZFS read/write ops"

This reverts commit b5eadc37f12c4b97a52705830d2d9097498049c2. It
introduces a deadlock as outlined in issue #504.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>