tools/blktap2: Fix use of uninitialised variable in _tap_list_join3()

Clang points out:

  tap-ctl-list.c:457:28: error: variable 'entry' is uninitialized when
  used here [-Werror,-Wuninitialized]
          for (; *_entry != NULL; ++entry) {
                                    ^~~~~

The content of that loop clearly was meant to iterate over _entry rather than
entry, so is fixed to do so.  This presumably fixes a memory leak when
tapdisks get orphed, as only the first item on the list got freed.

There is no use of entry at all.  It is referenced in a
list_for_each_entry(tl, &tap->list, entry) construct, but this is just a
member name, and not a reference to local scope variable of the same name.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>

tools/blktap2/control/tap-ctl-list.c

@@ -400,7 +400,7 @@ int
 _tap_list_join3(int n_minors, int *minorv, int n_taps, struct tapdisk *tapv,
 		tap_list_t ***_list)
 {
-	tap_list_t **list, **_entry, *entry;
+	tap_list_t **list, **_entry;
 	int i, _m, err;
 
 	list = tap_ctl_alloc_list(n_minors + n_taps);
@@ -454,7 +454,7 @@ _tap_list_join3(int n_minors, int *minorv, int n_taps, struct tapdisk *tapv,
 	}
 
 	/* free extraneous list entries */
-	for (; *_entry != NULL; ++entry) {
+	for (; *_entry != NULL; ++_entry) {
 		free_list(*_entry);
 		*_entry = NULL;
 	}