ALSA: seq: Add snd_seq_expand_var_event_at() helper

Create a new variant of snd_seq_expand_var_event() for expanding the
data starting from the given byte offset.  It'll be used by the new
UMP sequencer code later.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-19-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>