bio: don't write "bio: create slab" messages to syslog

When using device mapper, there are many "bio: create slab" messages in
the log. Device mapper targets have different front_pad, so each time when
we load a target that wasn't loaded before, we allocate a slab with the
appropriate front_pad and there is associated "bio: create slab" message.

This patch removes these messages, there is no need for them.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>