ocfs2: Create specific get_extent_tree functions.

A caller knows what kind of extent tree they have.  There's no reason
they have to call ocfs2_get_extent_tree() with a NULL when they could
just as easily call a specific function to their type of extent tree.

Introduce ocfs2_dinode_get_extent_tree(),
ocfs2_xattr_tree_get_extent_tree(), and
ocfs2_xattr_value_get_extent_tree().  They only take the necessary
arguments, calling into the underlying __ocfs2_get_extent_tree() to do
the real work.

__ocfs2_get_extent_tree() is the old ocfs2_get_extent_tree(), but
without needing any switch-by-type logic.

ocfs2_get_extent_tree() is now a wrapper around the specific calls.  It
exists because a couple alloc.c functions can take et_type.  This will
go later.

Another benefit is that ocfs2_xattr_value_get_extent_tree() can take a
struct ocfs2_xattr_value_root* instead of void*.  This gives us
typechecking where we didn't have it before.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
diff --git a/fs/ocfs2/alloc.h b/fs/ocfs2/alloc.h
index 23c695dd..5cc9a83 100644
--- a/fs/ocfs2/alloc.h
+++ b/fs/ocfs2/alloc.h
@@ -56,7 +56,7 @@
 				    u32 new_clusters,
 				    u8 flags,
 				    struct ocfs2_alloc_context *meta_ac,
-				    void *private);
+				    struct ocfs2_xattr_value_root *xv);
 int ocfs2_xattr_tree_insert_extent(struct ocfs2_super *osb,
 				   handle_t *handle,
 				   struct inode *inode,