/proc/*/environ: wrong placing of ptrace_may_attach() check

It's a bit dopey-looking and can permit a task to cause a pagefault in an mm
which it doesn't have permission to read from.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>