mm/slab.c: fix kernel-doc warning for __ksize()

Fix kernel-doc warning in mm/slab.c:

  mm/slab.c:4215: warning: Function parameter or member 'objp' not described in '__ksize'

Also add Return: documentation section for this function.

Link: http://lkml.kernel.org/r/68c9fd7d-f09e-d376-e292-c7b2bdf1774d@infradead.org
Fixes: 10d1f8cb

 ("mm/slab: refactor common ksize KASAN logic into slab_common.c")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>