bcachefs: Fix lost rebalance wakeups

Fix a missing wakeup in

'bcachefs set-file-option' -> xattr option update -> inode_write

this was missing because the wakeup needs to happen after transaction
commit. Also, add a 'kick' counter, to make sure we don't miss a wakeup
that occured right after we finished checking the rebalance_work btree.

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