The infiniband code includes its own lockdep compatibility code and this
#defines lockdep_set_class but doesn't include mutex_lock_nested() or
down_write_nested(), causing a compiler error in obdclass/llog_cat.c.

mutex_{un,}lock_nested() and down_{read,write}_nested() are functions
under CONFIG_DEBUG_LOCK_ALLOC, but #defined to regular locks without
CONFIG_DEBUG_LOCK_ALLOC so just checking #ifndef isn't sufficient.
b=16450