mm/vmalloc.c: fix "unused function" warning

compute_subtree_max_size() is unused, when building with
DEBUG_AUGMENT_PROPAGATE_CHECK=y.

  mm/vmalloc.c:785:1: warning: unused function 'compute_subtree_max_size' [-Wunused-function].

Link: https://lkml.kernel.org/r/20220129034652.75359-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>