posix_types: Make it possible to override __kernel_fsid_t

__kernel_fsid_t has members of type "long" on at least one
architecture (MIPS32), so make it possible to override the definition.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/1328677745-20121-3-git-send-email-hpa@zytor.com
Cc: Arnd Bergmann <arnd@arndb.de>