Blackfin arch: fixing bug - under IRQ stress, running applications may wrongly...

Blackfin arch: fixing bug - under IRQ stress, running applications may wrongly trigger an ICPLB miss and be killed

Disable IRQs while frobbing the CPLB registers, to avoid accessing the
data in current_rwx_mask while it isn't covered by CPLBs.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>