f2fs: fix potential overflow when adjusting GC cycle

commit b8c502b81e3f899c6488967dec61eed0f5907db3 upstream.

While comparing signed and unsigned variables, compiler will converts the
signed value to unsigned one, due to this reason, {in,de}crease_sleep_time
may return overflowed result.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>