SUNRPC: Use correct type in buffer length calculations

Use correct type signage in gss_krb5_remove_padding() when doing length
calculations.  Both xdr_buf.len and iov.iov_len are size_t, which is
unsigned; so use an unsigned type for our temporary length variable to
ensure we don't overflow it..

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>