kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE

Userspace has discovered the functionality offered by SYS_kcmp and has
started to depend upon it. In particular, Mesa uses SYS_kcmp for
os_same_file_description() in order to identify when two fd (e.g. device
or dmabuf) point to the same struct file. Since they depend on it for
core functionality, lift SYS_kcmp out of the non-default
CONFIG_CHECKPOINT_RESTORE into the selectable syscall category.

Rasmus Villemoes also pointed out that systemd uses SYS_kcmp to
deduplicate the per-service file descriptor store.

Note that some distributions such as Ubuntu are already enabling
CHECKPOINT_RESTORE in their configs and so, by extension, SYS_kcmp.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/3046
Signed-off-by: Chris Wilson <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Will Drewry <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Lucas Stach <[email protected]>
Cc: Rasmus Villemoes <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: [email protected]
Acked-by: Daniel Vetter <[email protected]> # DRM depends on kcmp
Acked-by: Rasmus Villemoes <[email protected]> # systemd uses kcmp
Reviewed-by: Cyrill Gorcunov <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Thomas Zimmermann <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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