ocfs2: Convenient access to an xattr bucket's header.

The xattr code often wants to access the ocfs2_xattr_header at the start
of an bucket.  Rather than walk the pointer chains, let's just create
another nice macro.  As a side benefit, we can get rid of the mostly
spurious ->bu_xh element on the bucket structure.  The idea is ripped
from the ocfs2_path code.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>