xen/conring: Write to console ring even if console lock is busted

xen/conring: Write to console ring even if console lock is busted

console_lock_busted gets set when an NMI/MCE/Double Fault handler decides to
bring Xen down in an emergency.  conring_puts() cannot block and does
not have problematic interactions with the console_lock.

Therefore, choosing to not put the string into the console ring simply means
that the kexec environment cant find any panic() message caused by an IST
interrupt, which is unhelpful for debugging purposes.

In the case that two pcpus fight with console_force_unlock(), having slightly
garbled strings in the console ring is far more useful than having nothing at
all.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Matt Wilson <msw@amazon.com>
Acked-by: Keir Fraser <keir@xen.org>