linprocfs: Fix some bugs in the maps file implementation.

• 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
Reviewed by: kib
MFC after: 1 week
MFC note: no OBJ_ANON in stable/12
Differential Revision: https://reviews.freebsd.org/D23062