ALSA: seq_oss: return full count for successful SEQ_FULLSIZE writes

ALSA: seq_oss: return full count for successful SEQ_FULLSIZE writes

snd_seq_oss_write() currently returns the raw load_patch() callback
result for SEQ_FULLSIZE events.

That callback is documented as returning 0 on success and -errno on
failure, but snd_seq_oss_write() is the file write path and should
report the number of user bytes consumed on success. Some in-tree
backends also return backend-specific positive values, which can still
be shorter than the original write size.

Return the full byte count for successful SEQ_FULLSIZE writes.
Preserve negative errors and convert any nonnegative completion to the
original count.

Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260324-alsa-seq-oss-fullsize-write-return-v1-1-66d448510538@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>