staging/lustre: switch from is_compat_task to in_compat_syscall

AFAICT, lustre is trying to determine syscall bitness.  Use the new
accessor.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>