bcachefs: create debugfs dir for each btree

bcachefs: create debugfs dir for each btree

This creates a subdirectory for each individual btree under the btrees/
debugfs directory.

Directory structure, before:

/sys/kernel/debug/bcachefs/$FS_ID/btrees/
├── alloc
├── alloc-bfloat-failed
├── alloc-formats
├── backpointers
├── backpointers-bfloat-failed
├── backpointers-formats
...

Directory structure, after:

/sys/kernel/debug/bcachefs/$FS_ID/btrees/
├── alloc
│   ├── bfloat-failed
│   ├── formats
│   └── keys
├── backpointers
│   ├── bfloat-failed
│   ├── formats
│   └── keys
...

Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>