slab: Fix off by one in object max number tests.

If freelist_idx_t is a byte, SLAB_OBJ_MAX_NUM should be 255 not 256, and
likewise if freelist_idx_t is a short, then it should be 65535 not
65536.

This was leading to all kinds of random crashes on sparc64 where
PAGE_SIZE is 8192.  One problem shown was that if spinlock debugging was
enabled, we'd get deadlocks in copy_pte_range() or do_wp_page() with the
same cpu already holding a lock it shouldn't hold, or the lock belonging
to a completely unrelated process.

Fixes: a41adfaa ("slab: introduce byte sized index for the freelist of a slab")
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>