bihash template: reinstate the check for the available memory in the arena

bihash template: reinstate the check for the available memory in the arena

ffb14b9554afa1e58c3657e0c91dda3135008274 has changed the semantics
of alloc_arena_next to become an offset off alloc_arena, but
in the available memory check in BV (alloc_aligned) it still treats
it as a virtual address, resulting in the check always succeeding,
thus over a prolonged period bihash arena allocator
potentially overwriting whatever is following the arena.

Change-Id: I18882c5f340ca767a389e15cca2696a0a97ef015
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>