ocfs2: Make ocfs2_extent_tree get/put instead of alloc.

Rather than allocating a struct ocfs2_extent_tree, just put it on the
stack.  Fill it with ocfs2_get_extent_tree() and drop it with
ocfs2_put_extent_tree().  Now the callers don't have to ENOMEM, yet
still safely ref the root_bh.

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