um: Fix incorrect assumptions about max pid length

pids are no longer limited to 16-bits, bump to 32-bits,
ie. 9 decimal characters.  Additionally sizeof("/") already
returns 2 - ie. it already accounts for trailing zero.

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Linux UM Mailing List <linux-um@lists.infradead.org>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>