ref_tracker: do xarray and workqueue job initializations earlier

The kernel test robot reported an oops that occurred when attempting to
deregister a dentry from the xarray during subsys_initcall().

The ref_tracker xarrays and workqueue job are being initialized in
late_initcall() which is too late. Move those to postcore_initcall()
instead.

Fixes: 65b584f5 ("ref_tracker: automatically register a file in debugfs for a ref_tracker_dir")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202506251406.c28f2adb-lkp@intel.com


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250626-reftrack-dbgfs-v1-1-812102e2a394@kernel.org