[SCSI] bsg: unexport sg v3 helper functions

blk_fill_sghdr_rq, blk_unmap_sghdr_rq, and blk_complete_sghdr_rq were
exported for bsg, however bsg was changed to support only sg v4.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>