mlx4_core: maintain available field in bitmap allocator

Add mlx4_bitmap_avail() to give the number of available resources. We want to
use this as a hint to whether to allocate a resources or not. This patch is
introduced to be used with allocation blue flame registers.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>