block: fix bogus gcc warning for uninitialized var usage

Newer gcc throw this warning:

        fs/bio.c: In function ?bio_alloc_bioset?:
        fs/bio.c:305: warning: ?p? may be used uninitialized in this function

since it cannot figure out that 'p' is only ever used if 'bs' is non-NULL.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>