ARM: smp: avoid incrementing mm_users on CPU startup

We should not be incrementing mm_users when we startup a secondary
CPU - doing so results in mm_users incrementing by one each time we
hotplug a CPU, which will eventually wrap, and will cause problems.

Other architectures such as x86 do not increment mm_users, but only
mm_count, so we follow that pattern.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>