PDU reduce number of mallocs/memcpys in iscsi_add_data()

This patch adds logarithmic malloc behaviour to iscsi_add_data().
Currently for each new call there is a new buffer allocated
and all old data is copied to the new buffer. Change this by
allocating at least PAGE_SIZE bytes and increase the allocation
by powers of 2 each time it does no longer fit.

include/iscsi.h

@@ -33,6 +33,7 @@ struct iscsi_context;
 struct sockaddr;
 
 #define MAX_STRING_SIZE (255)
+#define PAGE_SIZE (4096)
 
 /*
  * Syntax for normal and portal/discovery URLs.
@@ -533,6 +534,7 @@ iscsi_task_mgmt_target_cold_reset_async(struct iscsi_context *iscsi,
 
 struct iscsi_data {
        int size;
+       int alloc_size;
        unsigned char *data;
 };