sysctl: reject gigantic reads/write to sysctl files

Instead of triggering a WARN_ON deep down in the page allocator just
give up early on allocations that are way larger than the usual sysctl
values.

Fixes: 32927393 ("sysctl: pass kernel pointers to ->proc_handler")
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>