PCI: Fix "try" semantics of bus and slot reset

The commit referenced below introduced device locking around save and
restore of state for each device during a PCI bus "try" reset, making it
decidely non-"try" and prone to deadlock in the event that a device is
already locked.  Restore __pci_reset_bus() and __pci_reset_slot() to their
advertised locking semantics by pushing the save and restore functions into
the branch where the entire tree is already locked.  Extend the helper
function names with "_locked" and update the comment to reflect this
calling requirement.

Fixes: b014e96d

 ("PCI: Protect pci_error_handlers->reset_notify() usage with device_lock()")
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Sinan Kaya <okaya@kernel.org>