A few helpers are only used from blk-wbt.c, so move them there, and put
wbt_track() behind the CONFIG_BLK_WBT typedef. This is in preparation
for changing how the wbt flags are tracked.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>