Since all bio_split calls refer the same single bio_split_pool, the bio_split
function can use bio_split_pool directly instead of the mempool_t parameter;

then the mempool_t parameter can be removed from bio_split param list, and
bio_split_pool is only referred in fs/bio.c file, can be marked static.
Signed-off-by: NDenis ChengRq <crquan@gmail.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>