fuse: Add io-uring sqe commit and fetch support

This adds support for fuse request completion through ring SQEs
(FUSE_URING_CMD_COMMIT_AND_FETCH handling). After committing
the ring entry it becomes available for new fuse requests.
Handling of requests through the ring (SQE/CQE handling)
is complete now.

Fuse request data are copied through the mmaped ring buffer,
there is no support for any zero copy yet.

Signed-off-by: Bernd Schubert <[email protected]>
Reviewed-by: Pavel Begunkov <[email protected]> # io_uring
Reviewed-by: Luis Henriques <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>

Author: bsbernd