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>

fs/nfs/dir.c

@@ -2309,7 +2309,7 @@ static DEFINE_SPINLOCK(nfs_access_lru_lock);
 static LIST_HEAD(nfs_access_lru_list);
 static atomic_long_t nfs_access_nr_entries;
 
-static unsigned long nfs_access_max_cachesize = ULONG_MAX;
+static unsigned long nfs_access_max_cachesize = 4*1024*1024;
 module_param(nfs_access_max_cachesize, ulong, 0644);
 MODULE_PARM_DESC(nfs_access_max_cachesize, "NFS access maximum total cache length");