base: Add Allocator_avl::size_at function

This function provides a way to request the size of an previously
allocated block. It is useful to to ease the implementation of realloc
functionality based on Allocator_avl.

base/include/base/allocator_avl.h

@@ -231,6 +231,11 @@ namespace Genode {
 
 			void free(void *addr, size_t) { free(addr); }
 
+			/**
+			 * Return size of block at specified address
+			 */
+			size_t size_at(void const *addr) const;
+
 			/**
 			 * Return the memory overhead per Block
 			 *

base/src/base/allocator/allocator_avl.cc

@@ -339,6 +339,15 @@ void Allocator_avl_base::free(void *addr)
 }
 
 
+size_t Allocator_avl_base::size_at(void const *addr) const
+{
+	/* lookup corresponding block */
+	Block *b = _find_by_address(reinterpret_cast<addr_t>(addr));
+
+	return (b && b->used()) ? b->size() : 0;
+}
+
+
 bool Allocator_avl_base::any_block_addr(addr_t *out_addr)
 {
 	Block *b = _addr_tree.first();