kern/60084: swap encryption: per-page tracking seems overengineered

["imuwoto%gmail.com@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60084
>Category:       kern
>Synopsis:       swap encryption: per-page tracking seems overengineered
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Mar 15 03:40:00 +0000 2026
>Originator:     YAMAMOTO Takashi
>Release:        -current
>Organization:
>Environment:
amd64
>Description:
swap encryption maintains a bitmap to track per-page encryption
status. it's wasteful and unnecessarily complicate the implementation.
it would be simpler to let user decide it at swapon time.
after all, i suppose a user would either wants to encrypt all of the
swap, or none of them. not any middle of them.
that way the encryption key can be generated at swapon time too.

>How-To-Repeat:
code inspection
>Fix:
my suggestions:

* reject attempts to change vm.swap_encrypt sysctl if swap is configured.
* remove swd_encmap
* generate key on swapon