[RESEND] Add sched_set_batch() and run bcachefs-rebalance under SCHED_BATCH

[Florian Schmaus]

Resending the patchset which first adds the sched_set_batch(),
akin to the existing sched_set_normal() and sched_set_fifo_low()
functions. In the second patch, we use sched_set_batch() to run
bcachefs' rebalance thread under SCHED_BATCH.

I am resending the patchset [1,2], since the two patches of the set
where not send to the same recipients. This probably caused some
confusion.

1: https://lore.kernel.org/lkml/20241114210649.71377-1-flo@xxxxxxxxxxxx/
2: https://lore.kernel.org/lkml/20241114210649.71377-2-flo@xxxxxxxxxxxx/