uapi linux/coda.h: use __kernel_pid_t for userspace

Part of a patch by Mikko Rapeli, as Arnd Bergman commented on the
original patch.

   pid_t might differ between libc and the kernel, so the kernel
   interface has to use types that the kernel defines.

Link: http://lkml.kernel.org/r/f374a71f4d351bc8c8b3ac18ad7765c88d806d10.1558117389.git.jaharkes@cs.cmu.edu


Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: Zhouyang Jia <jiazhouyang09@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>