Since there are already a number of sites (ARM64, PowerPC) that
effectively nest nmi_enter(), lets make the primitive support this
before adding even more.
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>