Cleanup various build warnings (signed vs unsigned, unused vars,...)

Cleanup various build warnings (signed vs unsigned, unused vars,...)

* removed unused includes.
* removed unused parameters.
* Fixed a bunch of warnings around:
    int i;
    if (i < some_uint) ...
  and
    u32 x;
    ...
    if (x < 0) return error;
* Protect BLKDISCARD usage if it is not available.

Change-Id: Iede035b1beb2df01c961589a69aff47a5258ecd2
Signed-off-by: JP Abgrall <jpa@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>