jfs: Add missing mutex_unlock call to error path

Jan Kucera found an missing call to mutex_unlock() with his static code
checker.  It's an unlikely error path to hit in the real world, but it
should be fixed.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Reported-by: Jan Kucera <kucera.jan.cz@gmail.com>