bcachefs: Factor out progress.[ch]

bcachefs: Factor out progress.[ch]

the backpointers code has progress indicators; these aren't great, since
they print to the dmesg console and we much prefer to have progress
indicators reporting to a specific userspace program so they're not
spamming the system console.

But not all codepaths that need progress indicators support that yet,
and we don't want users to think "this is hung".

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