- With BCH_WRITE_SYNC, we no longer need the completion in struct
   dio_write
 - Pull out bch2_dio_write_copy_iov() into a separate non-inline
   function, it's code that doesn't run in the common case
 - Copy mapping and inode pointers into dio_write, avoiding pointer
   chasing at the start of bch2_dio_write_loop()
 - kthread_use_mm() is not needed in the common case; move it into
   bch2_dio_write_loop_async()
 - factor out various helpers from bch2_dio_write_loop() and rework
   control flow for better icache utilization

Other small optimizations:

 - bch2_keylist_free() is only used in one place, at the end of the
   bch2_write() path - drop the reinit
 - in bch2_disk_reservation_put(), check if res->sectors is nonzero
   before touching c->online_reserved, since that will likely be a cache
   miss
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: More DIO write path optimization

Better code prefetching (?)
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>