media: common: saa7146: Avoid a leak in vmalloc_to_sg()

Commit in Fixes turned a BUG() into a "normal" memory allocation failure.
While at it, it introduced a memory leak.
So fix it.

Also update the comment on top of the function to reflect what has been
change by the commit in Fixes.

Fixes: 40e986c9 ("media: common: saa7146: replace BUG_ON by WARN_ON")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>