manpages: Use canonical names for Linux filesystems

The manpages of linprocfs(5) and linsysfs(5) document incomplete names without
"fs" suffix. Make them identical to other filesystems.

PR: 283079
MFC after: 3 days
Reviewed by: ziaee, markj
Differential Revision: https://reviews.freebsd.org/D53155

(cherry picked from commit 88b9ca860ef332d4d32e97e0f8a166f12cffc8e3)