fiber: set fiber->csw = 0 for every creating fiber

It is strange to create a new fiber and see that it has yielded 100
times, when in fact it never actually did it.

The patch makes fiber->csw = 0 for each created fiber.

Follow-up #5799

changelogs/unreleased/gh-5799-set-csw-is-0-for-new-fibers.md

+## feature/fiber
+
+* Previously csw (Context SWitch) of new fiber can be more than 0, now it is always 0 (gh-5799).

src/lib/core/fiber.c

@@ -1260,6 +1260,7 @@ fiber_new_ex(const char *name, const struct fiber_attr *fiber_attr,
 	fiber->fid = cord->next_fid;
 	fiber_set_name(fiber, name);
 	register_fid(fiber);
+	fiber->csw = 0;
 
 	cord->next_fid++;
 	assert(cord->next_fid > FIBER_ID_MAX_RESERVED);

test/app/gh-5799-fiber-info.result

@@ -26,11 +26,9 @@ test_run:cmd('setopt delimiter ";"')
  | ...
 for i=1,100 do
     fibers[i] = fiber.new(function()
-        local start_csw = fiber.info()[fiber.self():id()].csw
         for j=1,10 do
             fiber.yield()
-            if j + start_csw == fiber.self():csw() and
-               j + start_csw == fiber.self():info().csw then
+            if j == fiber.self():csw() and j == fiber.self():info().csw then
                 csw_check_counter = csw_check_counter + 1
             end
         end

test/app/gh-5799-fiber-info.test.lua

@@ -10,11 +10,9 @@ fibers = {}
 test_run:cmd('setopt delimiter ";"')
 for i=1,100 do
     fibers[i] = fiber.new(function()
-        local start_csw = fiber.info()[fiber.self():id()].csw
         for j=1,10 do
             fiber.yield()
-            if j + start_csw == fiber.self():csw() and
-               j + start_csw == fiber.self():info().csw then
+            if j == fiber.self():csw() and j == fiber.self():info().csw then
                 csw_check_counter = csw_check_counter + 1
             end
         end