staging: lustre: replace OBD_SLAB_ALLOC_PTR with kmem_cache_alloc

The OBD_SLAB_ALLOC_PTR macro expands to call to kmem_cache_alloc,
which may be used directly.

Signed-off-by: Mike Rapoport <mike.rapoport@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>