linprocfs: Fix some inaccuracies in meminfo.
- Fill out MemFree correctly. Delete an ancient comment suggesting that we don't want to advertise the true quantity of free memory.
- Populate the Buffers field by reading vfs.bufspace.
- The page cache consists of all pages in page queues, not just the inactive queue.
PR: 248463
Reported and tested by: danfe
MFC after: 1 week
Sponsored by: The FreeBSD Foundation