nfs4: strengthen error check to avoid unexpected result

The variable error is ssize_t, which is signed and will
cast to unsigned when comapre with variable size, so add
a check to avoid unexpected result in case of negative
value of error.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>