[patch] MCA recovery module undefined symbol fix

The patch "MCA recovery improvements" added do_exit to mca_drv.c.
That's fine when the mca recovery code is built in the kernel
(CONFIG_IA64_MCA_RECOVERY=y) but breaks building the mca recovery
code as a module (CONFIG_IA64_MCA_RECOVERY=m).

Most users are currently building this as a module, as loading
and unloading the module provides a very convenient way to turn
on/off error recovery.

This patch exports do_exit, so mca_drv.c can build as a module.

Signed-off-by: Russ Anderson <(rja@sgi.com)>
Signed-off-by: Tony Luck <tony.luck@intel.com>