NFSv4: fix getacl head length estimation

Bitmap and attrlen follow immediately after the op reply header.  This
was an oversight from commit bf118a34.

Consequences of this are just minor efficiency (extra calls to
xdr_shrink_bufhead).

Fixes: bf118a34 "NFSv4: include bitmap in nfsv4 get acl data"
Reviewed-by: Kinglong Mee <kinglongmee@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>