staging: lustre: remove ll_invalidate_bdev()

It was a wrapper around invalidate_bdev(), totally useless.

Cc: Peng Tao <tao.peng@emc.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>