video: hyperv_fb: Avoid taking busy spinlock on panic path

The Hyper-V framebuffer code registers a panic notifier in order
to try updating its fbdev if the kernel crashed. The notifier
callback is straightforward, but it calls the vmbus_sendpacket()
routine eventually, and such function takes a spinlock for the
ring buffer operations.

Panic path runs in atomic context, with local interrupts and
preemption disabled, and all secondary CPUs shutdown. That said,
taking a spinlock might cause a lockup if a secondary CPU was
disabled with such lock taken. Fix it here by checking if the
ring buffer spinlock is busy on Hyper-V framebuffer panic notifier;
if so, bail-out avoiding the potential lockup scenario.

Cc: Andrea Parri (Microsoft) <[email protected]>
Cc: Dexuan Cui <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Michael Kelley <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Tianyu Lan <[email protected]>
Cc: Wei Liu <[email protected]>
Tested-by: Fabio A M Martins <[email protected]>
Signed-off-by: Guilherme G. Piccoli <[email protected]>
Reviewed-by: Michael Kelley <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Wei Liu <[email protected]>

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