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_setattr(struct xdr_stream *xdr)
{
	__be32 *p;
	uint32_t bmlen;
	int status;

	status = decode_op_hdr(xdr, OP_SETATTR);
	if (status)
		return status;
	p = xdr_inline_decode(xdr, 4);
	if (unlikely(!p))
		goto out_overflow;
	bmlen = be32_to_cpup(p);
	p = xdr_inline_decode(xdr, bmlen << 2);
	if (likely(p))
		return 0;
out_overflow:
	print_overflow_msg(__func__, xdr);
	return -EIO;
}

static int decode_setattr(struct xdr_stream *xdr)
{
	__be32 *p;
	uint32_t bmlen;
	int status;

	status = decode_op_hdr(xdr, OP_SETATTR);
	if (status)
		return status;
	p = xdr_inline_decode(xdr, 4);
	if (unlikely(!p))
		goto out_overflow;
	bmlen = be32_to_cpup(p);
	p = xdr_inline_decode(xdr, bmlen << 2);
	if (likely(p))
		return 0;
out_overflow:
	print_overflow_msg(__func__, xdr);
	return -EIO;
}
