ide-tape used to hit

[   58.614854] ide-tape: ht0: BUG: Two DSC requests queued!

due to the fact that another rq was being issued while the driver was
waiting for DSC to get set for the device executing ATAPI commands which
set the DSC to 1 to indicate completion.

Here's a sample output of that case:

issue REZERO_UNIT

[  143.088505] ide-tape: ide_tape_issue_pc: retry #0, cmd: 0x01
[  143.095122] ide: Enter ide_pc_intr - interrupt handler
[  143.096118] ide: Packet command completed, 0 bytes transferred
[  143.106319] ide-tape: ide_tape_callback: cmd: 0x1, dsc: 1, err: 0
[  143.112601] ide-tape: idetape_postpone_request: cmd: 0x1, dsc_poll_freq: 2000

we stall the ide-tape queue here waiting for DSC

[  143.119936] ide-tape: ide_tape_read_position: enter
[  145.119019] ide-tape: idetape_do_request: sector: 4294967295, nr_sectors: 0

and issue the new READ_POSITION rq and hit the check.

[  145.126247] ide-tape: ht0: BUG: Two DSC requests queued!
[  145.131748] ide-tape: ide_tape_read_position: BOP - No
[  145.137059] ide-tape: ide_tape_read_position: EOP - No

Also, ->postponed_rq used to point to that postponed request. To make
things worse, in certain circumstances the rq it was pointing to got
replaced unterneath it by swiftly reusing the same rq from the mempool
of the block layer practically confusing stuff even more.

However, we don't need to keep a pointer to that rq but simply wait for
DSC to be set first before issuing the follow-up request in the drive's
queue. In order to do that, we make idetape_do_request() first check the
DSC and if not set, we stall the drive queue giving the other device on
that IDE channel a chance.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>