cmd_mount: Fix test-only mount path

The comman line help claims that `bcachefs mount <DEV>` without a
mount point will do a dry-run mount - all the steps required to
mount the fs, but without actually doing the final real mount.

Make the code actually do this, rather than complain that you
haven't supplied a mountpoint if you don't provide a mountpoint

Signed-off-by: Christopher James Halse Rogers <raof@ubuntu.com>