nilfs-utils: move utility definitions into util.h

Gather up definitions of utility macros such as min_t(), max_t(),
DIV_ROUND_UP(), ARRAY_SIZE(), nilfs_cnt64_ge(), etc, into
include/util.h to make them reusable.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs-utils: use min_t() and max_t()

Simplify some calculations in lssu, lscp, lib/gc.c, lib/nilfs.c,
lib/cnoconv.c, and cleanerd with min_t() and max_t().

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>