An inotify watch retains the dentry used to add it even when its watch set
belongs to a shared inode. Allowing an unlinked gofer alias to be destroyed
while another hard link remains then leaves the watch targeting a dead
dentry that cannot be checkpointed.

Introduce WatchTarget and use the owning inode in gofer, tmpfs, EROFS, and
kernfs. Overlay and anonfs retain their dentry targets. Track inode aliases
in gofer and EROFS so removing the last watch rechecks every dentry kept out
of the eviction cache. Notify outside gofer's ancestry lock so one-shot
cleanup can safely evict other aliases.

Preserve gofer watches while another hard link represents the inode. Cover
unlinking the watched alias, checkpoint/restore, continued events through a
surviving link, and final watch deletion. Add alias cleanup tests for watch
removal, inotify close, and one-shot expiry.

Tested: affected filesystem unit tests and nogo; full inotify suites on
Linux, directfs, shared gofer, overlay, and tmpfs; focused save/restore and
lockdep tests.

Related to #16024.

Assisted-by: Codex