Revert "io_uring/msg_ring: kill alloc_cache for io_kiocb allocations"

This reverts commit 93e9d029 which is
commit df8922af 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: I3d8af605d9f2463a84319ac90ec5b38b7ef190f1
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>