sysfs: decouple IOKit snapshot allocation and traversal from g_lock

Holding g_lock across IOMalloc allocations, registry traversals, and memory
frees in sysfs_iokit.cpp causes severe lock contention and thread starvation
during frequent /sys filesystem operations.

Refactor sysfs_iokit.cpp to use a Read-Copy-Update (RCU) / Double-Checked Swap
pattern:
  - Encapsulate snapshot state (nodes, child indices, hash slots) inside
    a single heap-allocated struct sysfs_snapshot container.
  - Modify sysfs_snap_build() to construct and populate fresh snapshots in
    complete isolation without acquiring g_lock.
  - Update sysfs_snap_ensure() to execute memory allocation and IOKit tree
    traversals unlocked, re-acquiring g_lock only to perform an atomic pointer
    swap with double-checked TTL validation.
  - Deallocate stale snapshot memory after dropping g_lock.

This reduces g_lock hold times from millisecond-range tree walks to a few
nanoseconds for pointer swaps, eliminating reader starvation during parallel
vnode lookups and tree walks.

Author: somestupidgirl