NFS: advance nfs_entry cookie only after decoding completes successfully

In nfs[34]_decode_dirent, the cookie is advanced as soon as it is
read, but decoding may still fail later in the function, returning
an error.  Because the cookie has been advanced, the failing entry
is not re-requested from the server, resulting in a missing directory
entry.

In addition, nfs v3 and v4 read the cookie at different locations
in the xdr_stream, so the behavior of the two can be inconsistent.

Fix these by reading the cookie into a temporary variable, and
only advancing the cookie once the entire entry has been decoded
from the xdr_stream successfully.

Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>