NFSv4: fix getacl head length estimation

commit 6682c14bbe505a8b912c57faf544f866777ee48d upstream.

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>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>