bcachefs: Inline data extents

bcachefs: Inline data extents

This implements extents that have their data inline, in the value,
instead of the bkey value being pointers to the data - and the read and
write paths are updated to read from these new extent types and write
them out, when the write size is small enough.

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