block: remove unrelated header files and export symbol

block: remove unrelated header files and export symbol

Fix up the following items:

 - remove unrelated header files.
 - export interface function.
 - modify function cmdline_parts_parse return value, this will make
   it more friendly for the caller.

Signed-off-by: CaiZhiyong <[email protected]>
Cc: Ezequiel Garcia <[email protected]>
CC: Brian Norris <[email protected]>
Cc: "Wanglin (Albert)" <[email protected]>
Cc: Artem Bityutskiy <[email protected]>
Cc: Karel Zak <[email protected]>
Cc: Shmulik Ladkani <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>