vfs cache: mark vfs.cache.param.size as read-only

It was not meant to be writable and writes don't work correctly as they
fail to resize the hash.

(cherry picked from commit 33fdf1afaea931eaa5dd14e6c097d7ffafe2fb1a)
(cherry picked from commit 3f4e6aa2ae7c5d7b086f76e7ec445ec71a60a627)

Approved by: re (gjb)