mm/writeback: minor clean up for highmem_dirtyable_memory

mm/writeback: minor clean up for highmem_dirtyable_memory

Since commit a804552b9a15 ("mm/page-writeback.c: fix dirty_balance_reserve
subtraction from dirtyable memory"), local variable x can not be negative.
And it can not overflow when it is the total number of dirtyable highmem
pages.  Thus remove the unneeded comment and overflow check.

Link: https://lkml.kernel.org/r/20220224115416.46089-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>