mm, slab: cleanup slab_bug() parameters

slab_err() has variadic printf arguments but instead of passing them to
slab_bug() it does vsnprintf() to a buffer and passes %s, buf.

To allow passing them directly, turn slab_bug() to __slab_bug() with a
va_list parameter, and slab_bug() a wrapper with fmt, ... parameters.
Then slab_err() can call __slab_bug() without the intermediate buffer.

Also constify fmt everywhere, which also simplifies object_err()'s
call to slab_bug().

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>