x86, perf: P4 PMU -- use hash for p4_get_escr_idx()

Linear search over all p4 MSRs should be fine if only
we would not use it in events scheduling routine which
is pretty time critical. Lets use hashes. It should speed
scheduling up significantly.

v2: Steven proposed to use more gentle approach than issue
    BUG on error, so we use WARN_ONCE now

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Lin Ming <ming.m.lin@intel.com>
LKML-Reference: <20100512174242.GA5190@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>