Staging: lustre: Fix no use of assignment in if condition

This patch fixes the following checkpatch.pl error in lustre/ldlm/interval_tree.c-
ERROR: do not use assignment in if condition

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>