mm/slab: minor coding style tweaks

Fix some coding style issues, improve code reading.  Adds whitespace to
clearly separate the parameters.

Link: https://lkml.kernel.org/r/1612841499-32166-1-git-send-email-daizhiyuan@phytium.com.cn


Signed-off-by: Zhiyuan Dai <daizhiyuan@phytium.com.cn>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>