block/blk-lib.c: make __blkdev_issue_zeroout static

__blkdev_issue_zeroout is only used in blk-lib.c

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-lib.c

@@ -226,7 +226,7 @@ EXPORT_SYMBOL(blkdev_issue_write_same);
  *  Generate and issue number of bios with zerofiled pages.
  */
 
-int __blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
+static int __blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
 				  sector_t nr_sects, gfp_t gfp_mask)
 {
 	int ret;