zonefs: remove redundant null bio check

bio_alloc() with __GFP_DIRECT_RECLAIM, which is included in
GFP_NOFS, never fails, see comments in bio_alloc_bioset().

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>