ocfs2: Gell into ocfs2_xa_set()

ocfs2_xa_set() wraps the ocfs2_xa_prepare_entry()/ocfs2_xa_store_value()
logic.  Both callers can now use the same routine.  ocfs2_xa_remove()
moves directly into ocfs2_xa_set().

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