f2fs: convert discard_wake and gc_wake to bool type

discard_wake and gc_wake have only two values, 0 or 1.
So there is no need to use int type to store them.

BTW, move discard_wake to the end of the
discard_cmd_control structure.

Before:

  - sizeof(struct discard_cmd_control): 8392

After move:

  - sizeof(struct discard_cmd_control): 8384

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>