selinux: fix empty write to keycreate file

When sid == 0 (we are resetting keycreate_sid to the default value), we
should skip the KEY__CREATE check.

Before this patch, doing a zero-sized write to /proc/self/keycreate
would check if the current task can create unlabeled keys (which would
usually fail with -EACCESS and generate an AVC). Now it skips the check
and correctly sets the task's keycreate_sid to 0.

Bug report: https://bugzilla.redhat.com/show_bug.cgi?id=1719067



Tested using the reproducer from the report above.

Fixes: 4eb582cf ("[PATCH] keys: add a way to store the appropriate context for newly-created keys")
Reported-by: Kir Kolyshkin <kir@sacred.ru>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>