umtx: shm: Collapse USHMF_REG_LINKED and USHMF_OBJ_LINKED flags

...into the only USHMF_LINKED, as they are always set or unset together.

This is both to stop giving the impression that they can be set/unset
independently, which they can't with the current code, and to make it
clearer that an upcoming reference counting fix is correct.

Reviewed by: kib
Approved by: emaste (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46126

(cherry picked from commit dd83da532c36830a0c0aac624903849262ec6f68)