md/raid1: remove unnecessary null checking

If %__GFP_DIRECT_RECLAIM is set then bio_alloc_bioset will always
be able to allocate a bio. See comment of bio_alloc_bioset.

Signed-off-by: Gou Hao <gouhao@uniontech.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231214151458.28970-1-gouhao@uniontech.com