Blackfin arch: fix bug - MPU crashes under stress

Bug Description:
a customer reported under IRQ stress, running applications may
wrongly trigger an ICPLB miss and be killed. after playing a
bit more, here's a test case that may be triggering the same bug.

Fixing:
After modifying page protections, only modify the active CPLBs if the
protection change was made for the active mm.
Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>

include/asm-blackfin/mmu_context.h

@@ -173,8 +173,10 @@ static inline void protect_page(struct mm_struct *mm, unsigned long addr,
 
 static inline void update_protections(struct mm_struct *mm)
 {
+	if (mm->context.page_rwx_mask == current_rwx_mask) {
 		flush_switched_cplbs();
 		set_mask_dcplbs(mm->context.page_rwx_mask);
+	}
 }
 #endif