nfs: Fix GETATTR bitmap verification

When decoding GETATTR replies, the client checks the attribute bitmap
for which attributes the server has sent.  It misses bits at the word
boundaries, though; fix that.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>