bcachefs: Change allocations for ec stripes to blocking

bcachefs: Change allocations for ec stripes to blocking

We don't want writes to not get erasure coded just because the allocator
temporarily wasn't keeping up.

However, it's not guaranteed that these allocations will ever succeed,
we can currently get stuck - especially if devices are different sizes -
we still have work to do in this area.

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