f2fs: fix to initialize min_mtime with ULLONG_MAX

Since sit_i.min_mtime's type is unsigned long long, so we should
initialize it with max value of the type ULLONG_MAX instead of
LLONG_MAX.

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