GSSD: Pass GSS_context lifetime to the kernel.

GSSD: Pass GSS_context lifetime to the kernel.

From: Andy Adamson <andros@netapp.com>

The kernel gss_cl_ctx stores the context lifetime in gc_expiry, set
by gssd in do_downcall() called by process_krb5_upcall(). The lifetime
value is currently not related at all to the Kerberos TGS lifetime.
It is either set to the value of gssd -t <timeout>, or to a kernel
default of 3600 seconds.

Most of the time the gssd -t command line is not set, and a timeout
value of zero was sent to the kernel triggering the use of the 3600
second kernel default timeout.

In order for the kernel to properly know when to renew a context, or to
stop buffering writes for a context about to expire, the gc_expiry value
needs to reflect the credential lifetime used to create the context.

Note that gss_inquire_cred returns the number of seconds for which the
context remains valid in the lifetime_rec parameter.

Send the actual TGS remaining lifetime to the kernel. It can still be
overwritten by the gssd -t command line option, or set to the kernel
default if the gss_inquire_cred call fails (which sets the lifetime_rec
to zero).

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Steve Dickson <steved@redhat.com>