powerpc/hw_brk: Fix off by one error when validating DAWR region end

commit e2a800beaca1f580945773e57d1a0e7cd37b1056 upstream.

The Data Address Watchpoint Register (DAWR) on POWER8 can take a 512
byte range but this range must not cross a 512 byte boundary.

Unfortunately we were off by one when calculating the end of the region,
hence we were not allowing some breakpoint regions which were actually
valid.  This fixes this error.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Reported-by: Edjunior Barbosa Machado <emachado@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>