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>
__be32 * xdr_reserve_space(struct xdr_stream *xdr, size_t nbytes)
{
	__be32 *p = xdr->p;
	__be32 *q;

	/* align nbytes on the next 32-bit boundary */
	nbytes += 3;
	nbytes &= ~3;
	q = p + (nbytes >> 2);
	if (unlikely(q > xdr->end || q < p))
		return NULL;
	xdr->p = q;
	xdr->iov->iov_len += nbytes;
	xdr->buf->len += nbytes;
	return p;
}

__be32 * xdr_reserve_space(struct xdr_stream *xdr, size_t nbytes)
{
	__be32 *p = xdr->p;
	__be32 *q;

	/* align nbytes on the next 32-bit boundary */
	nbytes += 3;
	nbytes &= ~3;
	q = p + (nbytes >> 2);
	if (unlikely(q > xdr->end || q < p))
		return NULL;
	xdr->p = q;
	xdr->iov->iov_len += nbytes;
	xdr->buf->len += nbytes;
	return p;
}
