io_uring: add splice(2) support

*on top of for-5.6 + async patches*

Not the fastets implementation, but I'd need to stir up/duplicate splice.c bits to do it more efficiently.

note: rebase on top of the recent inflight patchset.

v2:
- u32 len and SQE layout changes (Jens)
- output file is in sqe->fd for automatic hash_reg_file support
- handle unbound_nonreg_file for the second fd
- file leaks fixed with REQ_F_NEED_CLEANUP
- place SPLICE_F_FD_IN_FIXED in splice flags (Jens)
- loff_t* -> loff_t, -1 means not specified offset

v3: [PATCH 3/3] changes
- fd u32 -> s32 (Stefan Metzmacher)
- add BUILD_BUG_SQE_ELEM() (Stefan Metzmacher)
- accept and ignore ioprio (Stefan Metzmacher)
- off_in -> splice_off_in

v4:
- rebase + a bit of function renaming
- make file_get/put accept req instead of ctx (Jens)
- fix lost REQ_F_FIXED_FILE

Pavel Begunkov (3):
splice: make do_splice public
io_uring: add interface for getting files
io_uring: add splice(2) support

 fs/io_uring.c                 | 181 ++++++++++++++++++++++++++++------
 fs/splice.c                   |   6 +-
 include/linux/splice.h        |   3 +
 include/uapi/linux/io_uring.h |  14 ++-
 4 files changed, 171 insertions(+), 33 deletions(-)

2.24.0