MINOR: shctx: Add a maximum object size parameter.

This patch adds a new parameter to shctx_init() function to be used to
limit the size of each shared object, -1 value meaning "no limit".
diff --git a/include/types/shctx.h b/include/types/shctx.h
index 186f736..53dca3f 100644
--- a/include/types/shctx.h
+++ b/include/types/shctx.h
@@ -40,6 +40,7 @@
 	struct list avail;  /* list for active and free blocks */
 	struct list hot;     /* list for locked blocks */
 	unsigned int nbav;  /* number of available blocks */
+	int max_obj_size;   /* maximum object size. */
 	void (*free_block)(struct shared_block *first, struct shared_block *block);
 	short int block_size;
 	unsigned char data[0];