io_uring: use files_lookup_fd_locked()

While valid we don't need to open-code rcu dereferences if we're
acquiring file_lock anyway.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20231010030615.GO800259@ZenIV


Signed-off-by: Christian Brauner <brauner@kernel.org>