feat(ublk): functional options for block size, queue depth, max IO

[arkamar]

Today ublk.New(backend, size) hard-codes 512-byte blocks, queue depth 128, and 128 KiB max IO. Downstream consumers (e.g. a rootfs provider using 4 KiB blocks for parity with an older NBD backend) need these configurable. Backward-compatible: existing 2-arg New(backend, size) callers are unchanged.

New(backend, size, opts ...Option) exposes the previously hard-coded knobs:

ublk.New(b, sz)                                  // defaults
ublk.New(b, sz, ublk.WithBlockSize(4096))        // 4 KiB LBA
ublk.New(b, sz,
    ublk.WithBlockSize(4096),
    ublk.WithQueueDepth(64),
    ublk.WithMaxIOSize(64<<10),
)

setParams derives LogicalBSShift / PhysicalBSShift / IOOptShift / IOMinShift from bits.TrailingZeros32(cfg.blockSize) rather than the hard-coded 9. DevSectors stays in 512-byte units since the kernel sector size is fixed.

Test plan

• Unit: TestNewValidation table covers zero size, non-multiple size, bad block size, oversized queue depth, max-IO/block-size mismatch.
• Integration: TestNewWithOptions round-trips 4 KiB blocks through BLKBSZGET, BLKSECTGET, and a pwrite/ReadAt comparison.
• Full integration suite green, make lint clean.

[cursor]

PR Summary

Medium Risk
Touches core device-creation parameters and kernel-facing SET_PARAMS values; misconfiguration could lead to IO failures or filesystem/mount issues, but validation and integration coverage reduce risk.

Overview
ublk.New now accepts functional options to configure block size, io_uring queue depth, and max IO size, replacing previously hard-coded defaults.

Device parameter programming is updated to derive the kernel block-size shift values from the configured block size, and new unit + integration tests are added to validate option constraints and exercise a full filesystem round-trip with a non-default block size.

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