RAS/tracing: Use trace_seq_buffer_ptr() helper instead of open coded

Use the helper function trace_seq_buffer_ptr() to get the current location
of the next buffer write of a trace_seq object, instead of open coding
it.

This facilitates the conversion of trace_seq to use seq_buf.
Tested-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Petr Mladek <pmladek@suse.cz>
Cc: Chen Gong <gong.chen@linux.intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

drivers/firmware/efi/cper.c

@@ -294,7 +294,7 @@ void cper_mem_err_pack(const struct cper_sec_mem_err *mem,
 const char *cper_mem_err_unpack(struct trace_seq *p,
 				struct cper_mem_err_compact *cmem)
 {
-	const char *ret = p->buffer + p->len;
+	const char *ret = trace_seq_buffer_ptr(p);
 
 	if (cper_mem_err_location(cmem, rcd_decode_str))
 		trace_seq_printf(p, "%s", rcd_decode_str);