[IA64] arch/ia64/sn/kernel/mca.c: undo lock when sn_oemdata can't be extended

Clean up locking state in failure path.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Cc: Jes Sorensen <jes@trained-monkey.org>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>