bcachefs: Start moving debug info from sysfs to debugfs

bcachefs: Start moving debug info from sysfs to debugfs

In sysfs, files can only output at most PAGE_SIZE. This is a problem for
debug info that needs to list an arbitrary number of times, and because
of this limit some of our debug info has been terser and harder to read
than we'd like.

This patch moves info about journal pins and cached btree nodes to
debugfs, and greatly expands and improves the output we return.

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