uapi: fix linux/raid/md_p.h userspace compilation error

Use __le32 and __le64 instead of u32 and u64.

This fixes klibc build error:
  In file included from /klibc/usr/klibc/../include/sys/md.h:30:0,
                   from /klibc/usr/kinit/do_mounts_md.c:19:
  /linux-next/usr/include/linux/raid/md_p.h:414:51: error: 'u32' undeclared here (not in a function)
    (PPL_HEADER_SIZE - PPL_HDR_RESERVED - 4 * sizeof(u32) - sizeof(u64))

Reported-by: Greg Thelen <gthelen@google.com>
Reported-by: Nigel Croxon <ncroxon@redhat.com>
Tested-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>