listio support


Summary:
-------
  
  The lio patch adds POSIX listio completion notification support. It builds
on support provided by the aio event patch and adds an IOCB_CMD_GROUP
command to sys_io_submit().

  The purpose of IOCB_CMD_GROUP is to group together the following requests in
the list up to the end of the list.

  As part of listio submission, the user process prepends to a list of requests
an empty special aiocb with an aio_lio_opcode of IOCB_CMD_GROUP, filling only
the aio_sigevent fields.



Details:
-------

  An IOCB_CMD_GROUP is added to the IOCB_CMD enum in include/linux/aio_abi.h

  A struct lio_event is added in include/linux/aio.h

  A struct lio_event *ki_lio is added to struct iocb in include/linux/aio.h


 In sys_io_submit(), upon detecting such an IOCB_CMD_GROUP marker iocb, an
lio_event is created in lio_create() which contains the necessary information
for signaling a thread (signal number, pid, notify type and value) along with
a count of requests attached to this event.

  The following depicts the lio_event structure:

        struct lio_event {
                atomic_t        lio_users;
                int             lio_wait;
                __s32           lio_pid;
                __u16           lio_signo;
                __u16           lio_notify;
                __u64           lio_value;
                uid_t           lio_uid, lio_euid;
        };

  lio_users holds a count of the number of requests attached to this lio. It
is incremented with each request submitted and decremented at each request
completion. Thread notification occurs when this count reaches 0.

  Each subsequent submitted request is attached to this lio_event by setting
the request kiocb->*ki_lio to that lio_event (in io_submit_one()) and
incrementing the lio_users count.

  In aio_complete(), if the request is attached to an lio (ki_lio <> 0), then
lio_check() is called to decrement the lio_users count and eventually signal
the user process when all the requests in the group have completed.


  The IOCB_CMD_GROUP command semantic is as follows:

       - if the associated aiocb sigevent is NULL then we want to group
         requests for the purpose of blocking on the group completion
         (LIO_WAIT sync behavior).

       - if the associated sigevent is valid (not NULL) then we want to
         group requests for the purpose of being notified upon that
         group of requests completion (LIO_NOWAIT async behaviour).