bcachefs: bcachefs_metadata_version_extent_flags

This implements a new extent field bitflags that apply to the whole
extent. There's been a couple things we've wanted this for in the past,
but the immediate need is extent poisoning, to solve a rebalance issue.

Unknown extent fields can't be parsed (we won't known their size, so we
can't advance to the next field), so this is an incompat feature, and
using it prevents the filesystem from being mounted by old versions.

This also adds the BCH_EXTENT_poisoned flag; this indicates that the
data is known to be bad (i.e. there was a checksum error, and we had to
write a new checksum) and reads will return errors.

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