aio poll, io_pgetevents and a new in-kernel poll API

Hi all,

this series adds support for the IOCB_CMD_POLL operation to poll for the
readyness of file descriptors using the aio subsystem.  The API is based
on patches that existed in RHAS2.1 and RHEL3, which means it already is
supported by libaio.  To implement the poll support efficiently new
methods to poll are introduced in struct file_operations:  get_poll_head
and poll_mask.  The first one returns a wait_queue_head to wait on
(lifetime is bound by the file), and the second does a non-blocking
check for the POLL* events.  This allows aio poll to work without
any additional context switches, unlike epoll.

To make the interface fully useful a new io_pgetevents system call is
added, which atomically saves and restores the signal mask over the
io_pgetevents system call.  It it the logical equivalent to pselect and
ppoll for io_pgetevents.

The corresponding libaio changes for io_pgetevents support and
documentation, as well as a test case will be posted in a separate
series.

The changes were sponsored by Scylladb, and improve performance
of the seastar framework up to 10%, while also removing the need
for a privileged SCHED_FIFO epoll listener thread.

The patches are on top of Als __poll_t annoations, so I've also
prepared a git branch on top of those here:

    git://git.infradead.org/users/hch/vfs.git aio-poll

Gitweb:

    http://git.infradead.org/users/hch/vfs.git/shortlog/refs/...

Libaio changes:

    http://git.infradead.org/users/hch/libaio.git/shortlog/re...

Seastar changes:

    https://github.com/avikivity/seastar/commits/aio