sparc64: Fix executable bit testing in set_pmd_at() paths.

This code was mistakenly using the exec bit from the PMD in all
cases, even when the PMD isn't a huge PMD.

If it's not a huge PMD, test the exec bit in the individual ptes down
in tlb_batch_pmd_scan().

Signed-off-by: David S. Miller <davem@davemloft.net>