NFSv4: Fix the acl cache size calculation

Currently, we do not take into account the size of the 16 byte
struct nfs4_cached_acl header, when deciding whether or not we should
cache the acl data.  Consequently, we will end up allocating an
8k buffer in order to fit a maximum size 4k acl.

This patch adjusts the calculation so that we limit the cache size
to 4k for the acl header+data.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>