bcachefs: Fix incorrect multiple dev check in journal write path

It's uncomon to have multiple devices with journalling only on a subset,
but can be specified with the 'data_allowed' option. We need to know if
we're doing data/metadata writes to multiple devices, as that requires
issuing flushes before the journal writes.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>