mm, slub: extract get_partial() from new_slab_objects()

The later patches will need more fine grained control over individual actions
in ___slab_alloc(), the only caller of new_slab_objects(), so this is a first
preparatory step with no functional change.

This adds a goto label that appears unnecessary at this point, but will be
useful for later changes.

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