bcachefs: Deferred btree updates

bcachefs: Deferred btree updates

Will be used in the future for inode updates, which will be very helpful
for multithreaded workloads that have to update the inode with every
extent update (appends, or updates that change i_sectors)

Also will be used eventually for fully persistent alloc info

However - we still need a mechanism for reserving space in the journal
prior to getting a journal reservation, so it's not technically safe to
make use of this just yet, we could deadlock with the journal full
(although not likely to be an issue in practice)

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