ARM: 7925/1: mm: keep track of last ASID allocation to improve bitmap searching

Since we only clear entries in the ASID bitmap on a rollover event, the
bitmap tends to consist of a block of consecutive set bits followed by
a block of consecutive clear bits. The exception to this rule is for
ASIDs which have been carried over from a previous generation, but
these are bound by the number of CPUs.

This patch optimises our bitmap searching strategy, so that we search
from the last successful allocation, rather than search from index 1
each time we allocate a new ASID.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>