bcachefs: Improvements to writing alloc info

bcachefs: Improvements to writing alloc info

Now that we've got transactional alloc info updates (and have for
awhile), we don't need to write it out on shutdown, and we don't need to
write it out on startup except when GC found errors - this is a big
improvement to mount/unmount performance.

This patch also fixes a few bugs where we weren't writing out alloc
info (on new filesystems, and new devices) and should have been.

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