io_uring/kbuf: move locking into io_kbuf_drop()

Move the burden of locking out of the caller into io_kbuf_drop(), that
will help with furher refactoring.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/530f0cf1f06963029399f819a9a58b1a34bebef3.1738724373.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>