mm/rmap.c: remove set but not used variable 'cstart'

Fixes gcc '-Wunused-but-set-variable' warning:

mm/rmap.c: In function page_mkclean_one:
mm/rmap.c:906:17: warning: variable cstart set but not used [-Wunused-but-set-variable]

It is not used any more since
commit cdb07bde

 ("mm/rmap.c: remove redundant variable cend")

Link: http://lkml.kernel.org/r/20190724141453.38536-1-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>