arm64: Prepare arch_nmi_enter() for recursion

When using nmi_enter() recursively, arch_nmi_enter() must also be recursion
safe. In particular, it must be ensured that HCR_TGE is always set while in
NMI context when in HYP mode, and be restored to it's former state when
done.

The current code fails this when interleaved wrong. Notably it overwrites
the original hcr state on nesting.

Introduce a nesting counter to make sure to store the original value.

Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Alexandre Chartre <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Catalin Marinas <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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