ARC: [mm] serious bug in vaddr based icache flush

vaddr used to index the cache was clipped from the wrong end, and thus
would potentially fail to flush the correct lines.

The problem was dorment for so long because up until the recent
optimizations it was only used for ptrace break-point only flushes.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>