NFS: Limit the size of the access cache by default

Currently, we have no real limit on the access cache size (we set it
to ULONG_MAX). That can lead to credentials getting pinned for a
very long time on lots of files if you have a system with a lot of
memory.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>