lru.h: Enforces the methods lru_pin() and lru_unpin()

If lru_*pin() is called twice, the counter will be incr/decr
incorrectly since it will count more/less pinned objects than
there is and so corrupts the balancing (lru_adjust()).

Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>
lru.h: Re-balance top/bot when pin/unpin an object

When lru_*pin() an object we need to re-adjust the
top/bot balance since the lru_num_pinned has been modified.

Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>