Avoid too much reserved_size for erofs image

After the erofs image is built, it won't be modified again. The reserve
room is only used for the hash footer. There is no need to set an
excessive margin.

Bug: 199248596
Test: erofs image size is smaller than before
Signed-off-by: Huang Jianan <huangjianan@oppo.com>
Change-Id: If2535b9ad2c2d60bea74feeca576a168387ff341