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

This reverts commit ed7535b1 which is
commit 6975c1a4 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: Id4baa489cc433ca9c984bb37e693364df3c6ed6a
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>