xen/pciback: don't call pcistub_device_put() under lock

pcistub_device_put() shouldn't be called under spinlock, as it can
sleep.

For this reason pcistub_device_get_pci_dev() needs to be modified:
instead of always calling pcistub_device_get() just do the call of
pcistub_device_get() only if it is really needed. This removes the
need to call pcistub_device_put().

Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/lkml/Y+JUIl64UDmdkboh@kadam/


Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Link: https://lore.kernel.org/r/20230328084549.20695-1-jgross@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>