nfsd: use RCU to protect fi_deleg_file

fi_deleg_file can be NULLed by put_deleg_file() when fi_delegees drops
to zero during delegation teardown (e.g. DELEGRETURN). Concurrent
accesses from workqueue callbacks -- such as CB_NOTIFY -- can
dereference a NULL pointer if they race with this teardown.

Annotate fi_deleg_file with __rcu and convert all accessors to use
proper RCU primitives:

- rcu_assign_pointer() / RCU_INIT_POINTER() for stores
- rcu_dereference_protected() for reads under fi_lock or where
  fi_delegees > 0 guarantees stability

This prepares for a subsequent patch that will use rcu_read_lock +
rcu_dereference + nfsd_file_get to safely acquire a reference from
the CB_NOTIFY callback path without holding fi_lock.

While converting the error-path lease teardown in nfsd_get_dir_deleg(),
also add a nfsd_fsnotify_recalc_mask() call after dropping the lease, to
match the success path and the equivalent teardown in
nfs4_unlock_deleg_lease(). Without it, a failure after the lease is set
leaves the inode's fsnotify mask reflecting a delegation that no longer
exists.

That teardown already unlocks against fi_deleg_file->nf_file rather than
this client's nf->nf_file; document why. The lease's flc_file is set to
fi_deleg_file in nfs4_alloc_init_lease(), which differs from nf when an
earlier client already holds a delegation on the same directory, and
generic_delete_lease() matches on flc_file -- unlocking the wrong file
would leak the lease on the inode.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260616-dir-deleg-v7-8-6cbc7eac0ade@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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