ocfs2: Wrap up the common use cases of ocfs2_new_path().

The majority of ocfs2_new_path() calls are:

	ocfs2_new_path(path_root_bh(otherpath),
		       path_root_el(otherpath));

Let's call that ocfs2_new_path_from_path().  The rest do similar things
from struct ocfs2_extent_tree.  Let's call those
ocfs2_new_path_from_et().  This will make the next change easier.

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