Revert "block: remove the write_hint field from struct request"

This reverts commit 3e12e8c2 which is
commit 61952bb7 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: Ifefd0168a7f8a8f5112b143781619285394bbca3
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>