kernel: memslab: Prevent pointer underflow in free list init

The "p -= slab->info.block_size;" is causing the "p" pointer
to be underflow in RX architecture case, where the RAM address
start from 0x0, in some case p minus block size make it underflow
This change implementation uses an index-based reverse loop to
safely iterate over each block from last to first.

Signed-off-by: Duy Nguyen <duy.nguyen.xa@renesas.com>