Revert "posix: use sys_sem instead of k_spinlock for pool synch"

This reverts commit 6e66aa1f.

And fixes an indentation issue in the original code,
which triggers checkpatch.

This PR:
https://github.com/zephyrproject-rtos/zephyr/pull/71718
introduced a regression breaking CI:
https://github.com/zephyrproject-rtos/zephyr/issues/71814


Let's just revert it until a better fix is done.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>