trelay: fix deadlock on remove

trelay: fix deadlock on remove

Upon writing to "remove" file, debugfs_remove_recursive() blocks while
holding rtnl_lock. This is because debugfs' file_ops callbacks are
executed in debugfs_use_file_*() context which prevents file removal.

Fix this by only flagging the device for removal and then do the cleanup
in file_ops.release callback which is executed out of that context.

Signed-off-by: Ali MJ Al-Nasrawy <alimjalnasrawy@gmail.com>
(cherry picked from commit c2635b871d1dd03a6608a9255222672decd49e09)