block: mark bio_split_pool static

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: Denis ChengRq <crquan@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/include/linux/bio.h b/include/linux/bio.h
index fe12d0f..fb97221 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -300,9 +300,7 @@
 	atomic_t			cnt;
 	int				error;
 };
-extern struct bio_pair *bio_split(struct bio *bi, mempool_t *pool,
-				  int first_sectors);
-extern mempool_t *bio_split_pool;
+extern struct bio_pair *bio_split(struct bio *bi, int first_sectors);
 extern void bio_pair_release(struct bio_pair *dbio);
 
 extern struct bio_set *bioset_create(int, int);