Fix some issues in the linprocfs maps file.

[markj]

Summary

• Export the offset into the backing object, not the object size.
• Fix a bug where we would print the previous entry's "offset" when a map_entry has no object.
• Try to identify shared mappings. Linux prints "s" when the mapping "may be shared". This attempt is not perfect, for example, we print "p" for anonymous memory that may be shared via minherit(INHERIT_SHARE).

PR: 240992