make powerpc BUG_ON() OK with pointers and bitwise

Since powerpc insists on printing the _value_ of condition
and on casting it to long...  At least let's make it a force-cast.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>