errinj: introduce delayed injection

With new macro ERROR_INJECT_COUNTDOWN it is possible to delay error
injection by iparam value: injection will be set only after iparam
times the path is executed. For instance:

void
foo(int i)
{
	/* 2 is delay counter. */
	ERROR_INJECT_COUNTDOWN(ERRINJ_FOO, {
		 printf("Error injection on %d cycle!\n", i);
		});
}

void
boo(void)
{
	for (int i = 0; i < 10; ++i)
		foo(i);
}

box.error.injection.set('ERRINJ_FOO', 2)

The result is "Error injection on 2 cycle!". This type of error
injection can turn out to be useful to set injection in the middle of
query processing. Imagine following scenario:

void
foo(void)
{
	int *fds[10];
	for (int i = 0; i < 10; ++i) {
		fds[i] = malloc(sizeof(int));
		if (fds[i] == NULL)
			goto cleanup;
	}
cleanup:
	free(fds[0]);
}

"cleanup" section obviously contains error and leads to memory leak.
But using means of casual error injection without delay such situation
can't be detected: OOM can be set only for first cycle iteration and in
this particular case no leaks take place.

Reviewed-by: Vladislav Shpilevoy <vshpilevoi@mail.ru>

src/lib/core/errinj.h

@@ -166,6 +166,7 @@ errinj_foreach(errinj_cb cb, void *cb_ctx);
 #  define ERROR_INJECT(ID, CODE)
 #  define ERROR_INJECT_WHILE(ID, CODE)
 #  define errinj(ID, TYPE) ((struct errinj *) NULL)
+#  define ERROR_INJECT_COUNTDOWN(ID, CODE)
 #else
 #  /* Returns the error injection by id */
 #  define errinj(ID, TYPE) \
@@ -184,6 +185,12 @@ errinj_foreach(errinj_cb cb, void *cb_ctx);
 		while (errinj(ID, ERRINJ_BOOL)->bparam) \
 			CODE; \
 	} while (0)
+#  define ERROR_INJECT_COUNTDOWN(ID, CODE)				\
+	do {								\
+		if (errinj(ID, ERRINJ_INT)->iparam-- == 0) {		\
+			CODE;						\
+		}							\
+	} while (0)
 #endif
 
 #define ERROR_INJECT_RETURN(ID) ERROR_INJECT(ID, return -1)