nilfs2: use GFP_NOIO for bio_alloc instead of GFP_NOWAIT

Alberto Bertogli advised me about bio_alloc() use in nilfs:
On Sat, 13 Jun 2009 22:52:40 -0300, Alberto Bertogli wrote:
> By the way, those bio_alloc()s are using GFP_NOWAIT but it looks
> like they could use at least GFP_NOIO or GFP_NOFS, since the caller
> can (and sometimes do) sleep. The only caller is nilfs_submit_bh(),
> which calls nilfs_submit_seg_bio() which can sleep calling
> wait_for_completion().

This takes in the comment and replaces the use of GFP_NOWAIT flag with
GFP_NOIO.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>