spin_lock may be an empty struct for !SMP configurations and so
arch_spin_is_locked may return unconditional 0 and trigger the VM_BUG_ON
even when the lock is held.

Replace spin_is_locked by lockdep_assert_held.  We will not BUG anymore
but it is questionable whether crashing makes a lot of sense in the
uncharge path.  Uncharge happens after the last page reference was
released so nobody should touch the page and the function doesn't update
any shared state except for res counter which uses synchronization of
its own.
Signed-off-by: NMichal Hocko <mhocko@suse.cz>
Reviewed-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>