LU-97 lustre-modules should depend on ldiskfs.

LU-97 lustre-modules should depend on ldiskfs.

Only for the server of course.
But depend on a generically named tag, and have ldiskfs provide
that tag so that any future backing stores can be used as a
replacement for ldiskfs.

Signed-off-by: Brian J. Murrell <brian@whamcloud.com>
Change-Id: Ibef2b8c2b2a8c7339143dc32d32b59ace94f00fd
Reviewed-on: http://review.whamcloud.com/265
Reviewed-by: Robert Read <rread@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>