powerpc: Fix kernel crash in show_instructions() w/DEBUG_VIRTUAL

With CONFIG_DEBUG_VIRTUAL=y, we can hit a BUG() if we take a hard
lockup watchdog interrupt when in OPAL mode.

This happens in show_instructions() if the kernel takes the watchdog
NMI IPI, or any other interrupt, with MSR_IR == 0. show_instructions()
updates the variable pc in the loop and the second iteration will
result in BUG().

We hit the BUG_ON due the below check in  __va()

  #define __va(x)
  ({
  	VIRTUAL_BUG_ON((unsigned long)(x) >= PAGE_OFFSET);
  	(void *)(unsigned long)((phys_addr_t)(x) | PAGE_OFFSET);
  })

Fix it by moving the check out of the loop. Also update nip so that
the nip == pc check still matches.

Fixes: 4dd7554a

 ("powerpc/64: Add VIRTUAL_BUG_ON checks for __va and __pa addresses")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
[mpe: Use IS_ENABLED(), massage change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200524093822.423487-1-aneesh.kumar@linux.ibm.com