sparc64: Fix page table walk for PUD hugepages

For a PUD hugepage entry, we need to propagate bits [32:22]
from virtual address to resolve at 4M granularity. However,
the current code was incorrectly propagating bits [29:19].
This bug can cause incorrect data to be returned for pages
backed with 16G hugepages.

Signed-off-by: Nitin Gupta <nitin.m.gupta@oracle.com>
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>