mm/vmalloc.c: use helper function va_size()

Use helper function va_size() to improve code readability. No functional
modification involved.

Link: https://lkml.kernel.org/r/20240906102539.3537207-1-zhangpeng362@huawei.com


Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>