tracing/synthetic: Skip first entry for stack traces

While debugging another issue I noticed that the stack trace output
contains the number of entries on top:

         <idle>-0       [000] d..4.   203.322502: wake_lat: pid=0 delta=2268270616 stack=STACK:
=> 0x10
=> __schedule+0xac6/0x1a98
=> schedule+0x126/0x2c0
=> schedule_timeout+0x242/0x2c0
=> __wait_for_common+0x434/0x680
=> __wait_rcu_gp+0x198/0x3e0
=> synchronize_rcu+0x112/0x138
=> ring_buffer_reset_online_cpus+0x140/0x2e0
=> tracing_reset_online_cpus+0x15c/0x1d0
=> tracing_set_clock+0x180/0x1d8
=> hist_register_trigger+0x486/0x670
=> event_hist_trigger_parse+0x494/0x1318
=> trigger_process_regex+0x1d4/0x258
=> event_trigger_write+0xb4/0x170
=> vfs_write+0x210/0xad0
=> ksys_write+0x122/0x208

Fix this by skipping the first element. Also replace the pointer
logic with an index variable which is easier to read.

Link: https://lkml.kernel.org/r/20230816154928.4171614-3-svens@linux.ibm.com

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 00cf3d67

 ("tracing: Allow synthetic events to pass around stacktraces")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>