Here, pmu1 cannot be released, and pmu2 cannot be reserved.
This patch modifies register_pmu to reject registrations where a PMU is
already present, preventing this problem. PMUs which can have multiple
instances should not use the PMU reservation framework.
Signed-off-by: Mark Rutland <mark.rutland@arm.com> Acked-by: Jamie Iles <jamie@jamieiles.com> Acked-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>