compat_sys_io_submit() cleanup

  Cleanup compat_sys_io_submit by duplicating some of the native syscall
logic in the compat layer and directly calling io_submit_one() instead
of fooling the syscall into thinking it is called from a native 64-bit
caller.

  This eliminates:

  - the overhead of copying the nr iocb pointers on the userspace stack

  - the PAGE_SIZE/(sizeof(void *)) limit on the number of iocbs that
    can be submitted.

  This is also needed for the completion notification patch to avoid having
to rewrite each iocb on the caller stack for io_submit_one() to find the
sigevents.