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 int decode_getfattr_generic(struct xdr_stream *xdr, struct nfs_fattr *fattr,
		struct nfs_fh *fh, const struct nfs_server *server, int may_sleep)
{
	__be32 *savep;
	uint32_t attrlen,
		 bitmap[3] = {0};
	int status;

	status = decode_op_hdr(xdr, OP_GETATTR);
	if (status < 0)
		goto xdr_error;

	status = decode_attr_bitmap(xdr, bitmap);
	if (status < 0)
		goto xdr_error;

	status = decode_attr_length(xdr, &attrlen, &savep);
	if (status < 0)
		goto xdr_error;

	status = decode_getfattr_attrs(xdr, bitmap, fattr, fh, server, may_sleep);
	if (status < 0)
		goto xdr_error;

	status = verify_attr_len(xdr, savep, attrlen);
xdr_error:
	dprintk("%s: xdr returned %d\n", __func__, -status);
	return status;
}

static int decode_getfattr_generic(struct xdr_stream *xdr, struct nfs_fattr *fattr,
		struct nfs_fh *fh, const struct nfs_server *server, int may_sleep)
{
	__be32 *savep;
	uint32_t attrlen,
		 bitmap[3] = {0};
	int status;

	status = decode_op_hdr(xdr, OP_GETATTR);
	if (status < 0)
		goto xdr_error;

	status = decode_attr_bitmap(xdr, bitmap);
	if (status < 0)
		goto xdr_error;

	status = decode_attr_length(xdr, &attrlen, &savep);
	if (status < 0)
		goto xdr_error;

	status = decode_getfattr_attrs(xdr, bitmap, fattr, fh, server, may_sleep);
	if (status < 0)
		goto xdr_error;

	status = verify_attr_len(xdr, savep, attrlen);
xdr_error:
	dprintk("%s: xdr returned %d\n", __func__, -status);
	return status;
}
