When unmounting an NFS mount using sec=krb5[ip], the umount system

When unmounting an NFS mount using sec=krb5[ip], the umount system
call could get hung sleeping on "gsssta" if the credentials for a user
that had been accessing the mount point have expired. This happened
because rpc_gss_destroy_context() would end up calling itself when the
"destroy context" RPC was attempted, trying to refresh the credentials.
This patch just checks for this case in rpc_gss_refresh() and returns
without attempting the refresh, which avoids the recursive call to
rpc_gss_destroy_context() and the subsequent hang.

Reviewed by:	dfr
Approved by:	re (Ken Smith), kib (mentor)