mm/slab_common: reduce an if statement in create_cache()

Move the 'out:' statement block out of the successful path to avoid
redundant check on 'err'. The value of 'err' is always zero on success
and negative on failure.

No functional changes, no performance improvements, just a little more
readability.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>