bcachefs: Fix extent merging

bcachefs: Fix extent merging

When merging extents, we have to check that we won't overflow size
fields in any CRC entries - but the check for this was wrong, because in
the loop it was in we weren't keeping a pointer to the (packed, encoded)
CRC field.

Fix this by moving it to its own loop.

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