[SCSI] block: make blk_rq_map_user take a NULL user-space buffer for WRITE

The commit 818827669d85b84241696ffef2de485db46b0b5e (block: make
blk_rq_map_user take a NULL user-space buffer) extended
blk_rq_map_user to accept a NULL user-space buffer with a READ
command. It was necessary to convert sg to use the block layer mapping
API.

This patch extends blk_rq_map_user again for a WRITE command. It is
necessary to convert st and osst drivers to use the block layer
apping API.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

fs/bio.c

diff --git a/fs/bio.c b/fs/bio.c
index 13be075..062299a 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -859,7 +859,7 @@
 	/*
 	 * success
 	 */
-	if (!write_to_vm) {
+	if (!write_to_vm && (!map_data || !map_data->null_mapped)) {
 		ret = __bio_copy_iov(bio, bio->bi_io_vec, iov, iov_count, 0, 0);
 		if (ret)
 			goto cleanup;