Blackfin arch: fix bug: kernel prints out error message twice

This fixes two things:
 - stop calling write_lock_irq/write_unlock_irq which can turn modify
   irq levels
 - don't calling mmput when handing exceptions - since this might_sleep,
   which does a rti, and leaves us in kernel space (irq15, rather
   than irq5).

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>