perf list: Give clues if failed to open tracing events directory

When executing the command "perf list", I met "Error: failed to open
tracing events directory" twice, the first reason is that there is no
"/sys/kernel/tracing/events" directory due to it does not enable the
kernel tracing infrastructure with CONFIG_FTRACE, the second reason
is that there is no root privileges.

Add the error string to tell the users what happened and what should
to do, and also call put_tracing_file() to free events_path a little
later to avoid messy code in the error message.

At the same time, just remove the redundant "/" of the file path in
the function get_tracing_file(), otherwise it shows something like
"/sys/kernel/tracing//events".

Before:

$ ./perf list
Error: failed to open tracing events directory

After:

(1) Without CONFIG_FTRACE

$ ./perf list
Error: failed to open tracing events directory
/sys/kernel/tracing/events: No such file or directory

(2) With CONFIG_FTRACE but no root privileges

$ ./perf list
Error: failed to open tracing events directory
/sys/kernel/tracing/events: Permission denied

Committer testing:

Redirect stdout to null to quickly test the patch:

Before:

$ perf list > /dev/null
Error: failed to open tracing events directory
$

After:

$ perf list > /dev/null
Error: failed to open tracing events directory
/sys/kernel/tracing/events: Permission denied
$

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/lkml/20240730062301.23244-3-yangtiezhu@loongson.cn
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>