erofs-utils: introduce fixed UNIX timestamp

Introduce option "-T" for UNIX timestamp.

Link: https://lore.kernel.org/r/20191012001847.GA5701@hsiangkao-HP-ZHAN-66-Pro-G1
Signed-off-by: Li Guifu <blucerlee@gmail.com>
[ Gao Xiang: use u64 for timestamp rather than long long. ]
Signed-off-by: Gao Xiang <hsiangkao@aol.com>