stackdepot: fix stack_depot_save_flags() in NMI context

In the Linux kernel, the following vulnerability has been resolved:

stackdepot: fix stack_depot_save_flags() in NMI context

Per documentation, stack_depot_save_flags() was meant to be usable from

NMI context if STACK_DEPOT_FLAG_CAN_ALLOC is unset. However, it still

would try to take the pool_lock in an attempt to save a stack trace in the

current pool (if space is available).

This could result in deadlock if an NMI is handled while pool_lock is

already held. To avoid deadlock, only try to take the lock in NMI context

and give up if unsuccessful.

The documentation is fixed to clearly convey this.