fiber: fix heap-buffer-overflow in fiber_stack_watermark_create

Fiber flags are initialized after fiber stack creation. As result
currently check for custom stack in fiber_stack_watermark_create does
not work. This leads to heap-buffer-overflow on putting watermark
if custom stack size is less than FIBER_STACK_SIZE_WATERMARK.

Close #9026

NO_DOC=bugfix

(cherry picked from commit 750e4890)