nfsd: close shrinker/GC/fsnotify vs per-net shutdown race in filecache

In the Linux kernel, the following vulnerability has been resolved:

nfsd: close shrinker/GC/fsnotify vs per-net shutdown race in filecache

The shrinker, GC worker, and fsnotify/lease callbacks can unhash an
nfsd_file from the rhashtable and then call
nfsd_file_dispose_list_delayed() to move it to the per-net dispose list.
If nfsd_file_cache_shutdown_net() runs concurrently, its rhashtable walk
misses the already-unhashed file, and its drain of the per-net dispose
list can run before the file has been queued. The file then sits on
the per-net list with no thread to drain it, leaking both the file and
its associated state.

The GC worker and shrinker already hold nfsd_gc_lock while walking the
LRU, but in the original code they release it before calling
nfsd_file_dispose_list_delayed(). The fsnotify/lease path
(nfsd_file_close_inode) has no synchronization at all.

Fix this by:

1. Widening nfsd_gc_lock in both nfsd_file_gc() and nfsd_file_lru_scan()
to cover the nfsd_file_dispose_list_delayed() call.

2. Wrapping nfsd_file_close_inode() in nfsd_gc_lock so that all three
callers of nfsd_file_dispose_list_delayed() hold the lock.

3. Adding a spin_lock/unlock(nfsd_gc_lock) barrier in
nfsd_file_cache_shutdown_net() after the purge, so that any
in-progress disposal has fully completed before the per-net list
is drained.

All operations inside the lock are non-sleeping (rhashtable lookups,
atomic bit/refcount ops, list moves, svc_wake_up), so the spinlock is
appropriate.