device: let online resize delegate shrink to kernel

[Komzpa]

This removes the tools-side online shrink precheck from bcachefs device resize so mounted filesystems can delegate smaller target sizes to the kernel resize ioctl.

That keeps current kernels honest: if shrink is unsupported, the ioctl still fails, now with shrink-specific context from the CLI.
The offline path still rejects shrinking because the bundled userspace/libbcachefs implementation has not gained the shrink machinery yet.

This is an independent tools-side delegation patch that matches the CLI boundary discussed around koverstreet/bcachefs#1070; it does not import or flatten the older shrink implementation branch. The full tools-targeted shrink stack is carried separately in #742 with original Jul Lang / Codex commits preserved where practical and Komzpa tools adaptations split at the tail.

Related to #268, koverstreet/bcachefs#781, koverstreet/bcachefs#1070, and koverstreet/bcachefs#1073.
Does not claim full filesystem shrinking support by itself.

Validation:

• BINDGEN=/home/kom/.cargo/bin/bindgen make -j32
• BINDGEN=/home/kom/.cargo/bin/bindgen cargo check -q
• git diff --check
• ./target/release/bcachefs device resize --help
• GitHub CI is passing for head 81ee809c6d8922a04c0e5d2cebbb19f7950ad47e

[koverstreet]

Cherry-picked to the testing branch as device: let online resize delegate shrink to kernel (2263343), authorship preserved. Thanks — right layering: the kernel is the authority on whether shrink is supported, so the online path should delegate rather than hardcode a userspace precheck. That also fixes the older-tool/newer-kernel case, where the hardcoded gate would wrongly block a shrink the loaded module supports, and uses the kernel's authoritative bucket count instead of the tool's cached usage. Verified the offline path keeps its own independent shrink guard (resize_offline still bails), and the kernel rejects online shrink before any mutation ('Cannot shrink yet'), so there's no data risk in the interim. Merging via cherry-pick rather than the merge button, so closing manually.