inotify: fix recursive directory rename handling

[allenanswerzq]

Fixes the Linux recursive-watcher failure described in rjeczalik#227 for the Syncthing fork.

Recursive watches maintain an internal tree of per-directory inotify watches. Directory removal, recreation, and rename events could be processed out of order, leaving watch descriptors associated with stale paths. Subsequent file events could therefore report the previous path or be dropped entirely.

This change:

• preserves inotify event ordering;
• serializes recursive-tree event dispatch;
• removes stale tree nodes on directory rename;
• maintains the tree when callers request native inotify events; and
• adds regression tests for a directory renamed away and back.

Verification:

• go test ./...
• go test -race ./...
• rename regression tests passed 30 repeated runs
• TestRecreated passed 30 repeated runs

Without the fix, the new regression tests consistently report events using the stale temporary/file path instead of original/file.

[allenanswerzq]

this pr should also fix this syncthing/syncthing#7950