powerpc/pkeys: Fix calculation of total pkeys.

Total number of pkeys calculation is off by 1. Fix it.

Fixes: 4fb158f6 ("powerpc: track allocation status of all pkeys")
Cc: stable@vger.kernel.org # v4.16+
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>