mm, slub: don't call flush_all() from slab_debug_trace_open()

slab_debug_trace_open() can only be called on caches with SLAB_STORE_USER flag
and as with all slub debugging flags, such caches avoid cpu or percpu partial
slabs altogether, so there's nothing to flush.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Christoph Lameter <cl@linux.com>