staging: lustre: Use proper constant types for L*_POISON values

On 32-bit m68k, I get lots of warnings like:

    warning: integer constant is too large for ‘long’ type

Switch the L*_POISON definitions from too-large constants and casts
to the proper constant types to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>