Non-functional cleanup of a "__user * filename"

The next patch defines a very similar interface, which I copied from
this definition.  Since I'm touching it anyway I don't see any reason
not to just go fix this one up.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Message-Id: <ef644540cfd8717f30bcc5e4c32f06c80b6c156e.1689092120.git.legion@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>