mkimg: Avoid leaking a page of mmap

[cperciva]

Summary

The image_file_map function adjusts the provided file offset to be
page-aligned, with a resulting increase in the size of the mapped
region; the increased size needs to be used when unmapping as well.

Reported by: Claude Opus 5.5
Fixes: baf4abfc39b2 ("Allow building mkimg as cross-tool")
Sponsored by: Amazon

[cperciva]

@sjg: Adding you since the commit I'm fixing is one of yours.