io_uring: add support for IORING_OP_MKDIRAT

IORING_OP_MKDIRAT behaves like mkdirat(2) and takes the same flags
and arguments.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dmitry Kadashev <dkadashev@gmail.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20210708063447.3556403-10-dkadashev@gmail.com


[axboe: add splice_fd_in check]
Signed-off-by: Jens Axboe <axboe@kernel.dk>