fs/devpts: always delete dcache dentry-s in dput()

d_delete only unhashes an entry if it is reached with
dentry->d_lockref.count != 1. Prior to commit 8ead9dd5 ("devpts:
more pty driver interface cleanups"), d_delete was called on a dentry
from devpts_pty_kill with two references held, which would trigger the
unhashing, and the subsequent dputs would release it.

Commit 8ead9dd5 reworked devpts_pty_kill to stop acquiring the second
reference from d_find_alias, and the d_delete call left the dentries
still on the hashed list without actually ever being dropped from dcache
before explicit cleanup. This causes the number of negative dentries for
devpts to pile up, and an `ls /dev/pts` invocation can take seconds to
return.

Provide always_delete_dentry() from simple_dentry_operations
as .d_delete for devpts, to make the dentry be dropped from dcache.

Without this cleanup, the number of dentries in /dev/pts/ can be grown
arbitrarily as:

`python -c 'import pty; pty.spawn(["ls", "/dev/pts"])'`

A syst...