util: use glibc macros to generate device numbers in the dev_t format

Our version of macroses are worng.

Our macros:
#define MINOR(dev)           ((dev) & 0xff)

Glibc function:
return (__dev & 0xff) | ((unsigned int) (__dev >> 12) & ~0xff);

Reported-by: Amey Deshpande <ameyd@google.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>