perf: Fix typos in sample code in the perf_event.h header

struct perf_event_mmap_page has members called "index" and
"cap_user_rdpmc".  Spell them correctly in the examples.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-api@vger.kernel.org
Link: http://lkml.kernel.org/r/320ba26391a8123cc16e5f02d24d34bd404332fd.1412313343.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/uapi/linux/perf_event.h

@@ -364,7 +364,7 @@ struct perf_event_mmap_page {
 	/*
 	 * Bits needed to read the hw events in user-space.
 	 *
-	 *   u32 seq, time_mult, time_shift, idx, width;
+	 *   u32 seq, time_mult, time_shift, index, width;
 	 *   u64 count, enabled, running;
 	 *   u64 cyc, time_offset;
 	 *   s64 pmc = 0;
@@ -383,11 +383,11 @@ struct perf_event_mmap_page {
 	 *       time_shift  = pc->time_shift;
 	 *     }
 	 *
-	 *     idx = pc->index;
+	 *     index = pc->index;
 	 *     count = pc->offset;
-	 *     if (pc->cap_usr_rdpmc && idx) {
+	 *     if (pc->cap_user_rdpmc && index) {
 	 *       width = pc->pmc_width;
-	 *       pmc = rdpmc(idx - 1);
+	 *       pmc = rdpmc(index - 1);
 	 *     }
 	 *
 	 *     barrier();
@@ -415,7 +415,7 @@ struct perf_event_mmap_page {
 	};
 
 	/*
-	 * If cap_usr_rdpmc this field provides the bit-width of the value
+	 * If cap_user_rdpmc this field provides the bit-width of the value
 	 * read using the rdpmc() or equivalent instruction. This can be used
 	 * to sign extend the result like:
 	 *
@@ -439,10 +439,10 @@ struct perf_event_mmap_page {
 	 *
 	 * Where time_offset,time_mult,time_shift and cyc are read in the
 	 * seqcount loop described above. This delta can then be added to
-	 * enabled and possible running (if idx), improving the scaling:
+	 * enabled and possible running (if index), improving the scaling:
 	 *
 	 *   enabled += delta;
-	 *   if (idx)
+	 *   if (index)
 	 *     running += delta;
 	 *
 	 *   quot = count / running;