Make the domain allocation size global to allow it to set in tests.

Some BSPs with multiple NIC locked in the tests due to not enough
memory. This provides a simple per BSP way to increase the memory
size.

rtemsbsd/include/rtems/bsd/bsd.h

@@ -111,6 +111,13 @@ typedef enum {
 	RTEMS_BSD_ALLOCATOR_DOMAIN_MALLOC
 } rtems_bsd_allocator_domain;
 
+/**
+ * @brief The size for the page/mbufs default allocator domain.
+ *
+ * Applications may set this value to change the value returned by the default.
+ */
+extern uintptr_t rtems_bsd_allocator_domain_page_mbuf_size;
+
 /**
  * @brief Returns the size for a specific allocator domain.
  *