NFSv4: include bitmap in nfsv4 get acl data

The NFSv4 bitmap size is unbounded: a server can return an arbitrary
sized bitmap in an FATTR4_WORD0_ACL request.  Replace using the
nfs4_fattr_bitmap_maxsz as a guess to the maximum bitmask returned by a server
with the inclusion of the bitmap (xdr length plus bitmasks) and the acl data
xdr length to the (cached) acl page data.

This is a general solution to commit e5012d1f "NFSv4.1: update
nfs4_fattr_bitmap_maxsz" and fixes hitting a BUG_ON in xdr_shrink_bufhead
when getting ACLs.

Fix a bug in decode_getacl that returned -EINVAL on ACLs > page when getxattr
was called with a NULL buffer, preventing ACL > PAGE_SIZE from being retrieved.

Cc: stable@kernel.org
Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
static void nfs4_locku_prepare(struct rpc_task *task, void *data)
{
	struct nfs4_unlockdata *calldata = data;

	if (nfs_wait_on_sequence(calldata->arg.seqid, task) != 0)
		return;
	if ((calldata->lsp->ls_flags & NFS_LOCK_INITIALIZED) == 0) {
		/* Note: exit _without_ running nfs4_locku_done */
		task->tk_action = NULL;
		return;
	}
	calldata->timestamp = jiffies;
	if (nfs4_setup_sequence(calldata->server,
				&calldata->arg.seq_args,
				&calldata->res.seq_res, 1, task))
		return;
	rpc_call_start(task);
}

static void nfs4_locku_prepare(struct rpc_task *task, void *data)
{
	struct nfs4_unlockdata *calldata = data;

	if (nfs_wait_on_sequence(calldata->arg.seqid, task) != 0)
		return;
	if ((calldata->lsp->ls_flags & NFS_LOCK_INITIALIZED) == 0) {
		/* Note: exit _without_ running nfs4_locku_done */
		task->tk_action = NULL;
		return;
	}
	calldata->timestamp = jiffies;
	if (nfs4_setup_sequence(calldata->server,
				&calldata->arg.seq_args,
				&calldata->res.seq_res, 1, task))
		return;
	rpc_call_start(task);
}
