dcache: add extra sanity checks of the dentry in dentry_free()

dcache: add extra sanity checks of the dentry in dentry_free()

If d_flags isn't what we expect, then it's good to display it. Add a new
DENTRY_WARN_ONCE() macro that also displays d_flags for the dentry.
Change D_FLAG_VERIFY() to call that instead of a generic WARN_ON_ONCE().

Change the existing hlist_unhashed() check in dentry_free() to use the
new macro, and add checks for other invariants of a dead dentry. Notably:

1) Ensure that DCACHE_LRU_LIST and DCACHE_SHRINK_LIST are not set.

2) Ensure that d_lockref is negative

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260422-dcache-warn-v1-1-50155e1b40b6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>