cxl/core/port: Use dedicated lock for decoder target list

Lockdep reports:

 ======================================================
 WARNING: possible circular locking dependency detected
 5.16.0-rc1+ #142 Tainted: G           OE
 ------------------------------------------------------
 cxl/1220 is trying to acquire lock:
 ffff979b85475460 (kn->active#144){++++}-{0:0}, at: __kernfs_remove+0x1ab/0x1e0

 but task is already holding lock:
 ffff979b87ab38e8 (&dev->lockdep_mutex#2/4){+.+.}-{3:3}, at: cxl_remove_ep+0x50c/0x5c0 [cxl_core]

...where cxl_remove_ep() is a helper that wants to delete ports while
holding a lock on the host device for that port. That sets up a lockdep
violation whereby target_list_show() can not rely holding the decoder's
device lock while walking the target_list. Switch to a dedicated seqlock
for this purpose.

Reported-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164367209095.208169.1171673319121271280.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>