mm/madvise.c: replace with page_size() in madvise_inject_error()

page_size() is supported after the commit a50b854e

 ("mm: introduce
page_size()").

Use page_size() in madvise_inject_error() for readability.

[akpm@linux-foundation.org: use ulong for `size', per David]
Link: http://lkml.kernel.org/r/29dce60c-38d6-0220-f292-e298f0c78c4d@huawei.com
Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Hu Shiyuan <hushiyuan@huawei.com>
Cc: Feilong Lin <linfeilong@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>