UBUNTU: SAUCE: shiftfs: add O_DIRECT support

UBUNTU: SAUCE: shiftfs: add O_DIRECT support

BugLink: https://bugs.launchpad.net/bugs/1837223
This enabled O_DIRECT support for shiftfs if the underlay supports it.

Currently shiftfs does not handle O_DIRECT if the underlay supports it.
This is blocking dqlite - an essential part of LXD - from profiting from
the performance benefits of O_DIRECT on suitable filesystems when used
with async io such as aio or io_uring.
Overlayfs cannot support this directly since the upper filesystem in
overlay can be any filesystem. So if the upper filesystem does not
support O_DIRECT but the lower filesystem does you're out of luck.
Shiftfs does not suffer from the same problem since there is not concept
of an upper filesystem in the same way that overlayfs has it.
Essentially, shiftfs is a transparent shim relaying everything to the
underlay while overlayfs' upper layer is not (completely).

Cc: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>