fiber: Pass a slab cache pointer when creating fiber stack

fiber_stack_destroy() doesn't depend on cord() helper but
rather accepts slab cache pointer in arguments, lets
make the same in fiber_stack_create() helper, otherwise
it looks quite imbalanced.

Signed-off-by: Kirill Yukhin <kyukhin@tarantool.org>
(cherry picked from commit fa6d78b397d375d4c18875ac72b3798b2e06a1e8)