bio: Fix outdated comment about bio_alloc_bioset()

In commit 451a9ebf bio_alloc_bioset()
was refactored not to take NULL as a valid argument for bs. This patch
changes the comment for that function accordingly. Currently, passing
NULL as argument to parameter bs would result in a NULL pointer
dereference.

Signed-off-by: Jaak Ristioja <ristioja@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>