kcov: do not instrument lib/stackdepot.c

There's no point in collecting coverage from lib/stackdepot.c, as it is
not a function of syscall inputs.  Disabling kcov instrumentation for that
file will reduce the coverage noise level.

Link: http://lkml.kernel.org/r/1474640972-104131-1-git-send-email-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Kostya Serebryany <kcc@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/Makefile

@@ -180,6 +180,7 @@ obj-$(CONFIG_IRQ_POLL) += irq_poll.o
 
 obj-$(CONFIG_STACKDEPOT) += stackdepot.o
 KASAN_SANITIZE_stackdepot.o := n
+KCOV_INSTRUMENT_stackdepot.o := n
 
 libfdt_files = fdt.o fdt_ro.o fdt_wip.o fdt_rw.o fdt_sw.o fdt_strerror.o \
 	       fdt_empty_tree.o