NFSv4.1 fix decode_free_stateid

The operation status is decoded in decode_op_hdr.

Stop the print_overflow message that is always hit without this patch:
nfs: decode_free_stateid: prematurely hit end of receive buffer. Remaining
buffer length is 0 words.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>