fs/notify: constify path

[ Upstream commit d5bf8889 ]

Reviewed-by: Matthew Bobrowski <repnop@google.com>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>