kernel: poll: patch recursive lock in z_vrfy_k_poll

In z_vrfy_k_poll, there is a memory access check
K_SYSCALL_MEMORY_WRITE which is wrapped in a spinlock, the same
spinlock used in z_handle_obj_poll_events which is called from
k_sem_give() for example.

The K_SYSCALL_MEMORY_WRITE() macro conditionally calls LOG_ERR()
which may call the UART console, which may call an API like
k_sem_give(). This will cause a deadlock since the locked spinlock
will be relocked, and a recursive lock if SPINLOCK_VALIDATE and
ASSERTS are enabled as the validation will fail, causing a LOG_ERR,
causing a k_sem_give() causing a relock... until stack overflows.

To solve the issue, only protect the copy of events to events_copy
with the spinlock, the content of events is not actually checked, and
bound is not shared, so there is no need to do this validation in a
critical section. The contents of events is shared so that must be
copied in atomically.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>