cachefiles: change cachefiles_bury_object to use start_renaming_dentry()

cachefiles: change cachefiles_bury_object to use start_renaming_dentry()

Rather then using lock_rename() and lookup_one() etc we can use
the new start_renaming_dentry().  This is part of centralising dir
locking and lookup so that locking rules can be changed.

Some error check are removed as not necessary.  Checks for rep being a
non-dir or IS_DEADDIR and the check that ->graveyard is still a
directory only provide slightly more informative errors and have been
dropped.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260224222542.3458677-11-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>