ARM: 7768/1: prevent risks of out-of-bound access in ASID allocator

On a CPU that never ran anything, both the active and reserved ASID
fields are set to zero. In this case the ASID_TO_IDX() macro will
return -1, which is not a very useful value to index a bitmap.

Instead of trying to offset the ASID so that ASID #1 is actually
bit 0 in the asid_map bitmap, just always ignore bit 0 and start
the search from bit 1. This makes the code a bit more readable,
and without risk of OoB access.

Cc: <stable@vger.kernel.org> # 3.9
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>