eventfd: only return events requested in poll_mask()

The ->poll_mask() operation has a mask of events that the caller
is interested in, but we're returning all events regardless.

Change to return only the events the caller is interested in. This
fixes aio IO_CMD_POLL returning immediately when called with POLLIN
on an eventfd, since an eventfd is almost always ready for a write.

Signed-off-by: Avi Kivity <avi@scylladb.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>