Revert "io_uring/kbuf: hold io_buffer_list reference over mmap"

This reverts commit 65938e81 which is
commit 561e4f94 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Id4642a514742ac9e809ac08a4cb9a9ace139402b
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>