io_uring: add sync_file_range and drains

In continuation of the fsync barrier patch from the other day, I
reworked that patch to turn it into a general primitive instead. This
means that any command can be flagged with IOSQE_IO_DRAIN, which will
insert a sequence point in the queue. If a request is marked with
IOSQE_IO_DRAIN, then previous commands must complete before this one
is issued. Subsequent requests are not started until the drain has
completed. The latter is a necessity since we track this through the
CQ index. If we allow later commands, then they could complete before
earlier commands and we'd mistakenly think that we have satisfied the
sequence point.

Patch 2 is just a prep patch for patch 3, which adds support for
sync_file_range() through io_uring. sync_file_range() is heavily used
by RocksDB.

Patches are also in my io_uring-next branch;

git://git.kernel.dk/linux-block io_uring-next

 fs/io_uring.c                 | 142 +++++++++++++++++++++++++++++++++-
 fs/sync.c                     | 135 +++++++++++++++++---------------
 include/linux/fs.h            |   3 +
 include/uapi/linux/io_uring.h |   3 +
 4 files changed, 216 insertions(+), 67 deletions(-)

-- 
Jens Axboe