devcgroup: always show positive major/minor num

 # echo "b $((0x7fffffff)):$((0x80000000)) rwm" > devices.allow
 # cat devices.list
 b 214748364:-21474836 rwm

though a major/minor number of 0x800000000 is meaningless, we
should not cast it to a negative value.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>