vfs: replace WARN(down_read_trylock, ...) abuse with proper asserts

Note the macro used here works regardless of LOCKDEP.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240602123720.775702-1-mjguzik@gmail.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>