hier(7): remove entry for non-existent /usr/compat

This is historical (?), but today /compat is the default according to
linux(4). The only remaining reference to /usr/compat in the src tree is
under tools/test/stress2.

Add a next-level entry for /compat/linux.

PR: 261349
Reviewed by: grahamperrin, karels, dchagin
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40876