NLM/NFSD: Fix lock notifications for async-capable filesystems

Instead of checking just the exportfs flag, use the new
locks_can_async_lock() helper which allows NLM and NFSD to once again
support lock notifications for all filesystems which use posix_lock_file().

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Link: https://lore.kernel.org/r/865c40da44af67939e8eb560d17a26c9c50f23e0.1726083391.git.bcodding@redhat.com


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>