target: do not submit a zero-bio I/O request

scsi_setup_fs_cmnd does not like to receive requests with no
bios attached to it. Special-case zero-length reads and writes,
by not submitting any bio.

Testcase: sg_raw /dev/sdb 28 00 00 00 00 00 00 00 00 00
should not fail
panics with the rest of the series but not this patch
behaves correctly without or with this series

Signed-off-by: Paolo Bonzini <[e-mail]>
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/target_core_iblock.c

···
 		goto fail;
 	cmd->priv = ibr;
 
+	if (!sgl_nents) {
+		atomic_set(&ibr->pending, 1);
+		iblock_complete_cmd(cmd);
+		return 0;
+	}
+
 	bio = iblock_get_bio(cmd, block_lba, sgl_nents);
 	if (!bio)
 		goto fail_free_ibr;