f2fs: fix wrong max cost initialization

This patch fixes missing increased max cost caused by a patch that we increased
cose of data segments in greedy algorithm.

Cc: <stable@vger.kernel.org> # v4.10+
Fixes: b9cd2061 "f2fs: node segment is prior to data segment selected victim"
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>