staging: lustre: Use is_vmalloc_addr

Instead of manually checking the bounds of VMALLOC_START and
VMALLOC_END, just use is_vmalloc_addr. That's what the function
was designed for.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>