s3:modules: add vfs_io_uring module

s3:modules: add vfs_io_uring module

The module makes use of the new io_uring infrastructure
(intruduced in linux 5.1), see https://lwn.net/Articles/778411/ and
http://git.kernel.dk/cgit/liburing/

Currently this only implements SMB_VFS_{PREAD,PWRITE,FSYNC}_SEND/RECV
and avoids the overhead of our userspace threadpool.

In future we'll hopefully make more use of more advanced io_uring
features.

For now we don't have automated tests as our test infrastructure
doesn't use a recent kernel. At least we're able to do compile tests
on fedora31.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Sat Feb 15 11:37:45 UTC 2020 on sn-devel-184