cxl/memdev: Fix endpoint port removal

cxl/memdev: Fix endpoint port removal

Testing of ram region support [1], stimulates a long standing bug in
cxl_detach_ep() where some cxl_ep_remove() cleanup is skipped due to
inability to walk ports after dports have been unregistered. That
results in a failure to re-register a memdev after the port is
re-enabled leading to a crash like the following:

    cxl_port_setup_targets: cxl region4: cxl_host_bridge.0:port4 iw: 1 ig: 256
    general protection fault, ...
    [..]
    RIP: 0010:cxl_region_setup_targets+0x897/0x9e0 [cxl_core]
    dev_name at include/linux/device.h:700
    (inlined by) cxl_port_setup_targets at drivers/cxl/core/region.c:1155
    (inlined by) cxl_region_setup_targets at drivers/cxl/core/region.c:1249
    [..]
    Call Trace:
     <TASK>
     attach_target+0x39a/0x760 [cxl_core]
     ? __mutex_unlock_slowpath+0x3a/0x290
     cxl_add_to_region+0xb8/0x340 [cxl_core]
     ? lockdep_hardirqs_on+0x7d/0x100
     discover_region+0x4b/0x80 [cxl_port]
     ? __pfx_discover_region+0x10/0x10 [cxl_port]
     device_for_each_child+0x58/0x90
     cxl_port_probe+0x10e/0x130 [cxl_port]
     cxl_bus_probe+0x17/0x50 [cxl_core]

Change the port ancestry walk to be by depth rather than by dport. This
ensures that even if a port has unregistered its dports a deferred
memdev cleanup will still be able to cleanup the memdev's interest in
that port.

The parent_port->dev.driver check is only needed for determining if the
bottom up removal beat the top-down removal, but cxl_ep_remove() can
always proceed given the port is pinned. That is, the two sources of
cxl_ep_remove() are in cxl_detach_ep() and cxl_port_release(), and
cxl_port_release() can not run if cxl_detach_ep() holds a reference.

Fixes: 2703c16c75ae ("cxl/core/port: Add switch port enumeration")
Link: http://lore.kernel.org/r/167564534874.847146.5222419648551436750.stgit@dwillia2-xfh.jf.intel.com
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/167601992789.1924368.8083994227892600608.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>