This patch converts ext3 to use the new export_operations for
exporting a filesystem.
In particular it
 - defines "ext3_get_parent" to do a lookup(".."),
 - calls d_splice_alias when ext3_lookup finds something,
   so that build a dcache path from the bottom up works, and
 - sets sb->s_export_op to point to an (almost empty)
   export_operations structure.  The fact that it is mostly empty
   means that the default lookup operations are used, which match the
   previous approach (i.e. use iget, and then check the generation
   number).