io_uring: batch task work locking

Many task_work handlers either grab ->uring_lock, or may benefit from
having it. Move locking logic out of individual handlers to a lazy
approach controlled by tctx_task_work(), so we don't keep doing
tons of mutex lock/unlock.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d6a34e147f2507a2f3e2fa1e38a9c541dcad3929.1629286357.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>