fix(ublk): prevent buffer slice UB by returning Option

[jaredLunde]

This PR fixes a potential undefined behavior in the ublk buffer pool where PoolSlot::as_mut_slice used debug_assert! before std::slice::from_raw_parts_mut. In release builds, a malformed kernel length exceeding SLOT_SIZE would create an out-of-bounds mutable slice.

Changes:

• Modify PoolSlot::as_mut_slice to return Option<&mut [u8]>, returning None for oversized requests
• Modify IoBuf::as_mut_slice to return Option<&mut [u8]>, using safe get_mut for heap variant
• Add early guard in acquire_io_buf to reject oversized lengths
• Update call site in glidefs/src/block/ublk/device.rs to handle None by logging and returning -EIO
• Add regression tests: pool_slot_rejects_oversized_slice and acquire_io_buf_rejects_oversized_len

The caller already has an error path for buffer allocation failures; this change routes oversized requests through the same degrade-safe path instead of risking UB.