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 __read_bytes_from_xdr_buf(struct xdr_buf *subbuf, void *obj, unsigned int len)
{
	unsigned int this_len;

	this_len = min_t(unsigned int, len, subbuf->head[0].iov_len);
	memcpy(obj, subbuf->head[0].iov_base, this_len);
	len -= this_len;
	obj += this_len;
	this_len = min_t(unsigned int, len, subbuf->page_len);
	if (this_len)
		_copy_from_pages(obj, subbuf->pages, subbuf->page_base, this_len);
	len -= this_len;
	obj += this_len;
	this_len = min_t(unsigned int, len, subbuf->tail[0].iov_len);
	memcpy(obj, subbuf->tail[0].iov_base, this_len);
}

static void __read_bytes_from_xdr_buf(struct xdr_buf *subbuf, void *obj, unsigned int len)
{
	unsigned int this_len;

	this_len = min_t(unsigned int, len, subbuf->head[0].iov_len);
	memcpy(obj, subbuf->head[0].iov_base, this_len);
	len -= this_len;
	obj += this_len;
	this_len = min_t(unsigned int, len, subbuf->page_len);
	if (this_len)
		_copy_from_pages(obj, subbuf->pages, subbuf->page_base, this_len);
	len -= this_len;
	obj += this_len;
	this_len = min_t(unsigned int, len, subbuf->tail[0].iov_len);
	memcpy(obj, subbuf->tail[0].iov_base, this_len);
}
