Treat GSSAPI error codes as unsigned.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
GSSAPI error codes (major and minor) are defined as unsigned values.
However, we treat them as signed while passing them down to the
kernel where conversion fails if they include the minus sign.
Convert them as unsigned.
Signed-off-by: Neil Brown <neilb@suse.de>