bcachefs: Don't disallow btree writes to RO devices

bcachefs: Don't disallow btree writes to RO devices

There's an inherent race with setting devices RO when they have dirty
btree nodes on them. We already check if a btree node is on an RO device
before we dirty it, so this patch just allows those writes so that we
don't have errors forcing the entire filesystem read only when trying to
remove a device.

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