bcachefs: Add .to_text() methods for all superblock sections

bcachefs: Add .to_text() methods for all superblock sections

This patch improves the superblock .to_text() methods and adds methods
for all types that were missing them. It also improves printbufs by
allowing them to specfiy what units we want to be printing in, and adds
new wrapper methods for unifying our kernel and userspace environments.

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