nfscl: Fix GetACL so that it does not want a reply cached

Without this patch, an NFSv4.1/4.2 GetACL RPC requests that
the session cache the reply. In some cases, the reply may
be too large to cache, resulting in a NFS4ERR_X
error from the server.

Since a GetACL is idempotent, disable reply caching for it,
by setting that it can generate a large reply.
Tested against a Linux server with a large ACL on a file.

MFC after: 2 weeks