up_switch_context() and up_exit() released the critical section and then
kept using the outgoing task's stack: a call/ret through
nxsched_switch_context() and the call into x86_64_fullcontextrestore().
Once the lock is released the outgoing task can be woken and run by
another CPU on that same stack, so those accesses race with it.
Release the critical section as the last step and enter
x86_64_fullcontextrestore() with a jmp so nothing is read from or
written to the outgoing stack after the release.
Assisted-by: Claude Code
Signed-off-by: raiden00pl <raiden00@railab.me>
dcca9a4735