Don't truncate /proc/PID/environ at 4096 characters

/proc/PID/environ currently truncates at 4096 characters, patch based on
the /proc/PID/mem code.

Signed-off-by: James Pearson <james-p@moving-picture.com>
Cc: Anton Arapov <aarapov@redhat.com>
Cc: Jan Engelhardt <jengelh@computergmbh.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>