base: add Slab::avail_entries accessor

This accessor is useful to eagerly expand the slab with new slab blocks,
side stepping the slab's built-in policy for the allocation of new slab
blocks.

This is particularly important when using the slab for allocating the
cap space meta-data for the base-hw kernel. To guarantee that the slab
gets never exhausted in the kernel, it is expanded before entering the
kernel.

repos/base/include/base/slab.h

@@ -109,6 +109,11 @@ class Genode::Slab : public Allocator
 		 */
 		static size_t entry_costs(size_t slab_size, size_t block_size);
 
+		/**
+		 * Return number of unused slab entries
+		 */
+		size_t avail_entries() const { return _total_avail; }
+
 		/**
 		 * Add new slab block as backing store
 		 *