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>