alpha: perf: fix out-of-bounds array access triggered from raw event

Vince's perf fuzzer uncovered the following issue on Alpha:

Unable to handle kernel paging request at virtual address fffffbfe4e46a0e8
CPU 0 perf_fuzzer(1278): Oops 0
pc = [<fffffc000031fbc0>]  ra = [<fffffc000031ff54>]  ps = 0007    Not tainted
pc is at alpha_perf_event_set_period+0x60/0xf0
ra is at alpha_pmu_enable+0x1a4/0x1c0
v0 = 0000000000000000  t0 = 00000000000fffff  t1 = fffffc007b3f5800
t2 = fffffbff275faa94  t3 = ffffffffc9b9bd89  t4 = fffffbfe4e46a098
t5 = 0000000000000020  t6 = fffffbfe4e46a0b8  t7 = fffffc007f4c8000
s0 = 0000000000000000  s1 = fffffc0001b0c018  s2 = fffffc0001b0c020
s3 = fffffc007b3f5800  s4 = 0000000000000001  s5 = ffffffffc9b9bd85
s6 = 0000000000000001
a0 = 0000000000000006  a1 = fffffc007b3f5908  a2 = fffffbfe4e46a098
a3 = 00000005000108c0  a4 = 0000000000000000  a5 = 0000000000000000
t8 = 0000000000000001  t9 = 0000000000000001  t10= 0000000027829f6f
t11= 0000000000000020  pv = fffffc000031fb60  at = fffffc0000950900
gp = fffffc0000940900  sp = fffffc007f4cbca8
Disabling lock debugging due to kernel taint
Trace:
[<fffffc000031ff54>] alpha_pmu_enable+0x1a4/0x1c0
[<fffffc000039f4e8>] perf_pmu_enable+0x48/0x60
[<fffffc00003a0d6c>] __perf_install_in_context+0x15c/0x230
[<fffffc000039d1f0>] remote_function+0x80/0xa0
[<fffffc00003a0c10>] __perf_install_in_context+0x0/0x230
[<fffffc000037b7e4>] smp_call_function_single+0x1b4/0x1d0
[<fffffc000039bb70>] task_function_call+0x60/0x80
[<fffffc00003a0c10>] __perf_install_in_context+0x0/0x230
[<fffffc000039bb44>] task_function_call+0x34/0x80
[<fffffc000039d3fc>] perf_install_in_context+0x9c/0x150
[<fffffc00003a0c10>] __perf_install_in_context+0x0/0x230
[<fffffc00003a5100>] SYSC_perf_event_open+0x360/0xac0
[<fffffc00003110c4>] entSys+0xa4/0xc0

This is due to the raw event encoding being used as an index directly
into the ev67_mapping array, rather than being validated against the
ev67_pmc_event_type enumeration instead. Unlike other architectures,
which allow raw events to propagate into the hardware counters with
little interference, the limited number of events on Alpha and the
strict event <-> counter relationships mean that raw events actually
correspond to the Linux-specific Alpha events, rather than anything
defined by the architecture.

This patch adds a new callback to alpha_pmu_t for validating the raw
event encoding with the Linux event types for the PMU, preventing the
out-of-bounds array access.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Michael Cree <mcree@orcon.net.nz>
Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>