selftests/tracing: Fix test_multiple_writes stall

When /sys/kernel/tracing/buffer_size_kb is less than 12KB,
the test_multiple_writes test will stall and wait for more
input due to insufficient buffer space.

Check current buffer_size_kb value before the test. If it is
less than 12KB, it temporarily increase the buffer to 12KB,
and restore the original value after the tests are completed.

Link: https://lore.kernel.org/r/[e-mail]
Fixes: 37f46601383a ("selftests/tracing: Add basic test for trace_marker_raw file")
Suggested-by: Steven Rostedt <[e-mail]>
Signed-off-by: Fushuai Wang <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/test.d/00basic/trace_marker_raw.tc

···
 	# The id must be four bytes, test that 3 bytes fails a write
 	if echo -n abc > ./trace_marker_raw ; then
 		echo "Too small of write expected to fail but did not"
+		echo ${ORIG} > buffer_size_kb
 		exit_fail
 	fi
 
···
 
 	if write_buffer 0xdeadbeef $size ; then
 		echo "Too big of write expected to fail but did not"
+		echo ${ORIG} > buffer_size_kb
 		exit_fail
 	fi
 }
 
+ORIG=`cat buffer_size_kb`
+
+# test_multiple_writes test needs at least 12KB buffer
+NEW_SIZE=12
+
+if [ ${ORIG} -lt ${NEW_SIZE} ]; then
+	echo ${NEW_SIZE} > buffer_size_kb
+fi
+
 test_buffer
-test_multiple_writes
+if ! test_multiple_writes; then
+	echo ${ORIG} > buffer_size_kb
+	exit_fail
+fi
+
+echo ${ORIG} > buffer_size_kb