scripts/decode_stacktrace.sh: guess basepath if not specified

Guess path to kernel sources using known location of symbol "kernel_init".
Make basepath argument optional.

Before:

$ echo 'vfs_open+0x0/0x0' | ./scripts/decode_stacktrace.sh vmlinux ""
vfs_open (home/khlebnikov/src/linux/fs/open.c:912)

After:

$ echo 'vfs_open+0x0/0x0' | ./scripts/decode_stacktrace.sh vmlinux
vfs_open (fs/open.c:912)
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Sasha Levin <sashal@kernel.org>
Link: http://lkml.kernel.org/r/159282922803.248444.2379229451667913634.stgit@buzzSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/decode_stacktrace.sh

@@ -3,14 +3,14 @@
 # (c) 2014, Sasha Levin <sasha.levin@oracle.com>
 #set -x
 
-if [[ $# < 2 ]]; then
+if [[ $# < 1 ]]; then
 	echo "Usage:"
-	echo "	$0 [vmlinux] [base path] [modules path]"
+	echo "	$0 <vmlinux> [base path] [modules path]"
 	exit 1
 fi
 
 vmlinux=$1
-basepath=$2
+basepath=${2-auto}
 modpath=$3
 declare -A cache
 declare -A modcache
@@ -152,6 +152,14 @@ handle_line() {
 	echo "${words[@]}" "$symbol $module"
 }
 
+if [[ $basepath == "auto" ]] ; then
+	module=""
+	symbol="kernel_init+0x0/0x0"
+	parse_symbol
+	basepath=${symbol#kernel_init (}
+	basepath=${basepath%/init/main.c:*)}
+fi
+
 while read line; do
 	# Let's see if we have an address in the line
 	if [[ $line =~ \[\<([^]]+)\>\] ]] ||