Conditional statements are faster than indirect calls. Use a member variable
to track the SCSI command submitter such that later patches can call
scsi_done(scmd) instead of scmd->scsi_done(scmd).

The asymmetric behavior that scsi_send_eh_cmnd() sets the submission
context to the SCSI error handler and that it does not restore the
submission context to the SCSI core is retained.

Cc: Hannes Reinecke <hare@suse.com>
Cc: Ming Lei <redacted>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/scsi/scsi_error.c | 18 +++++++-----------
 drivers/scsi/scsi_lib.c   |  9 +++++++++
 drivers/scsi/scsi_priv.h  |  1 +
 include/scsi/scsi_cmnd.h  |  7 +++++++
 4 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 572673873ddf..ba6d748a0246 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c

@@ -1577,6 +1577,15 @@ static blk_status_t scsi_prepare_cmd(struct request *req)
 
 static void scsi_mq_done(struct scsi_cmnd *cmd)
 {
+	switch (cmd->submitter) {
+	case BLOCK_LAYER:
+		break;
+	case SCSI_ERROR_HANDLER:
+		return scsi_eh_done(cmd);
+	case SCSI_RESET_IOCTL:
+		return;
+	}
+