include/linux/slab.h: fix a typo error in comment

fix a typo error in slab.h
"allocagtor" -> "allocator"

Signed-off-by: tangjianqiang <tangjianqiang@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Souptick Joarder <jrdr.linux@gmail.com>
Link: https://lkml.kernel.org/r/1600230053-24303-1-git-send-email-tangjianqiang@xiaomi.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>