bcachefs: bch2_extent_ptr_desired_durability()

bcachefs: bch2_extent_ptr_desired_durability()

This adds a new helper for getting a pointer's durability irrespective
of the device state, and uses it in the the data update path.

This fixes a bug where we do a data update but request 0 replicas to be
allocated, because the replica being rewritten is on a device marked as
failed.

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