scripts/decode_stacktrace.sh: add '-h' flag

When no parameters are passed, the usage instructions are presented only
when debuginfod-find is not found. This makes sense because with
debuginfod none of the positional parameters are needed. However it means
that users having debuginfod-find installed will have no chance of reading
the usage text without opening the file.

Many programs have a '-h' flag to get the usage, so add such a flag.
Invoking 'scripts/decode_stacktrace.sh -h' will now show the usage text
and exit.

Link: https://lkml.kernel.org/r/20240[e-mail]
Signed-off-by: Luca Ceresoli <[e-mail]>
Reviewed-by: Stephen Boyd <[e-mail]>
Cc: Alexis Lothoré (eBPF Foundation) <[e-mail]>
Cc: Konstantin Khlebnikov <[e-mail]>
Cc: Sasha Levin <[e-mail]>
Cc: Thomas Petazzoni <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

scripts/decode_stacktrace.sh

···
 	echo "Usage:"
 	echo "	$0 -r <release>"
 	echo "	$0 [<vmlinux> [<base_path>|auto [<modules_path>]]]"
+	echo "	$0 -h"
 }
 
 # Try to find a Rust demangler
···
 ADDR2LINE=${UTIL_PREFIX}addr2line${UTIL_SUFFIX}
 NM=${UTIL_PREFIX}nm${UTIL_SUFFIX}
 
-if [[ $1 == "-r" ]] ; then
+if [[ $1 == "-h" ]] ; then
+	usage
+	exit 0
+elif [[ $1 == "-r" ]] ; then
 	vmlinux=""
 	basepath="auto"
 	modpath=""