mm: Fix warning on make htmldocs caused by slab.c

This patch fixed following errors while make htmldocs
Warning(/mm/slab.c:1956): No description found for parameter 'page'
Warning(/mm/slab.c:1956): Excess function parameter 'slabp' description in 'slab_destroy'

Incorrect function parameter "slabp" was set instead of "page"

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>