Make BUG_ON() clang-compatible
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 <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>
Signed-off-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>

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