Create "export_operations" interface for filesystems to describe
whether and how they should be exported.

- add new field in struct super_block "s_export_op" to describe
  how a filesystem is exported (i.e. how filehandles are mapped to
  dentries).
- New module: fs/exportfs for holding helper code for mapping between
  filehandles and dentries
- Change nfsd to use new interface if it exists.
- Change ext2 to provide new interface
- Add documention to filesystems/Exporting

If s_export_op isn't set, old mechanism still works, but it is
planned to remove old method and only use s_export_op.