fs: change d_delete semantics

Change d_delete from a dentry deletion notification to a dentry caching
advise, more like ->drop_inode. Require it to be constant and idempotent,
and not take d_lock. This is how all existing filesystems use the callback
anyway.

This makes fine grained dentry locking of dput and dentry lru scanning
much simpler.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 163c890..746055b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2198,7 +2198,7 @@
 	return __d_cft(file->f_dentry);
 }
 
-static int cgroup_delete_dentry(struct dentry *dentry)
+static int cgroup_delete_dentry(const struct dentry *dentry)
 {
 	return 1;
 }