x86 MCA: Fix a deadlock in MCA exception processing

x86 MCA: Fix a deadlock in MCA exception processing

In exceptional circumstances, an MCA exception will trigger when the
freelist is exhausted. In such a case, no error will be logged on the list
and 'mca_count' will not be incremented.

Prior to this patch, all CPUs that received the exception would spin
forever.

With this change, the CPU that detects the error but finds the freelist
empty will proceed to panic the machine, ending the deadlock.

A follow-up to r260457.

Reported by:	Ryan Libby <rlibby at gmail.com>
Reviewed by:	jhb@
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D10536