target: Allow for target_submit_cmd() returning errors

We want it to be possible for target_submit_cmd() to return errors up
to its fabric module callers.  For now just update the prototype to
return an int, and update all callers to handle non-zero return values
as an error.

This is immediately useful for tcm_qla2xxx to fix a long-standing active
I/O session shutdown race, but tcm_fc, usb-gadget, and sbp-target the
fabric maintainers need to check + ACK that handling a target_submit_cmd()
failure due to session shutdown does not introduce regressions

(nab: Respin against for-next after initial NACK + update docbook comment +
      fix double se_cmd init in exception path for usb-gadget)

Cc: Chad Dupuis <[email protected]>
Cc: Arun Easi <[email protected]>
Cc: Chris Boot <[email protected]>
Cc: Stefan Richter <[email protected]>
Cc: Mark Rustad <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Felipe Balbi <[email protected]>
Cc: Andy Grover <[email protected]>
Signed-off-by: Roland Dreier <[email protected]>
Signed-off-by: Nicholas Bellinger <[email protected]>

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