bitmap_*() should cope with zero size bitmaps

bitmap_*() should cope with zero size bitmaps

... to match expectations set by memset()/memcpy().

Similarly for find_{first,next}_{,zero_}_bit() on x86.

__bitmap_shift_{left,right}() would also need fixing (they more
generally can't cope with the shift count being larger than the bitmap
size, and they perform undefined operations by possibly shifting an
unsigned long value by BITS_PER_LONG bits), but since these functions
aren't really used anywhere I wonder if we wouldn't better simply get
rid of them.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>