ocfs2: Introduce ocfs2_xa_loc

The ocfs2 extended attribute (xattr) code is very flexible.  It can
store xattrs in the inode itself, in an external block, or in a tree of
data structures.  This allows the number of xattrs to be bounded by the
filesystem size.

However, the code that manages each possible storage location is
different.  Maintaining the ocfs2 xattr code requires changing each hunk
separately.

This patch is the start of a series introducing the ocfs2_xa_loc
structure.  This structure wraps the on-disk details of an xattr
entry.  The goal is that the generic xattr routines can use
ocfs2_xa_loc without knowing the underlying storage location.

This first pass merely implements the basic structure, initializing it,
and wiping the name+value pair of the entry.

Signed-off-by: Joel Becker <joel.becker@oracle.com>