Make BUG_ON() clang-compatible

When trying to compile the beast with clang, it complains:

====
./include/lock.h:33:2: error: indirection of non-volatile null pointer will be deleted, not trap
        BUG_ON(ret < 0);
        ^~~~~~~~~~~~~~~
In file included from restorer.c:18:
./include/util.h:118:27: note: instantiated from:
#define BUG_ON(condition)       BUG_ON_HANDLER((condition))
                                ^
./include/util.h:100:4: note: instantiated from:
                        *(unsigned long *)NULL = 0xdead0000 + __LINE__; \
                        ^
====

Make clang happy again by adding 'volatile'.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>