Unionfs: lookup overhaul using vfs_path_lookup

Unionfs: lookup overhaul using vfs_path_lookup

Rework the lookup code to use vfs_path_lookup as much as possible, to ensure
that we have a vfsmount at this critical stage.  This is necessary for the
upcoming VFS API change from vfs_* to path_* methods.  This change also
allows unionfs to cross bind mounts and other mounts on lower branches.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>