linprocfs: Add /proc/vm/max_map_count

On Linux this limits the number of maps per mm struct.
We don't limit mappings, return a suitable large value.

Reviewed by: emaste
Differential revision: https://reviews.freebsd.org/D35351
MFC after: 2 weeks

(cherry picked from commit 7e2a451119373b7d3f2b3fd062d4ccb099dde022)