NFSv4: Fix pointer arithmetic in decode_getacl

Resetting the cursor xdr->p to a previous value is not a safe
practice: if the xdr_stream has crossed out of the initial iovec,
then a bunch of other fields would need to be reset too.

Fix this issue by using xdr_enter_page() so that the buffer gets
page aligned at the bitmap _before_ we decode it.

Also fix the confusion of the ACL length with the page buffer length
by not adding the base offset to the ACL length...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index c77d296..286ab70 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -3819,7 +3819,7 @@
 	if (ret)
 		goto out_free;
 
-	acl_len = res.acl_len - res.acl_data_offset;
+	acl_len = res.acl_len;
 	if (acl_len > args.acl_len)
 		nfs4_write_cached_acl(inode, NULL, 0, acl_len);
 	else