um: adjust size of pid_buf

Linux can have pids up to 4*1024*1024.  To handle such huge numbers
pid_buf needs to be larger.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>