scsi: always increment reference count

James reported:
> After e513cc1c module: Remove stop_machine from module unloading,
> module_refcount() is returning (unsigned long)-1 when called from within
> a routine that runs in module_exit.  This is confusing the scsi device
> put code which is coded to detect a module_refcount() of zero for
> running within a module exit routine and not try to do another
> module_put.  The fix is to restore the original behaviour of
> module_refcount() and return zero if we're running inside an exit
> routine.

The correct fix is to turn try_module_get() into __module_get(), and
always do the module_put().

Acked-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>