powerpc/64: Only WARN if __pa()/__va() called with bad addresses

We added checks to __pa() / __va() to ensure they're only called with
appropriate addresses. But using BUG_ON() is too strong, it means
virt_addr_valid() will BUG when DEBUG_VIRTUAL is enabled.

Instead switch them to warnings, arm64 does the same.

Fixes: 4dd7554a

 ("powerpc/64: Add VIRTUAL_BUG_ON checks for __va and __pa addresses")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220406145802.538416-5-mpe@ellerman.id.au