Btrfs: add skip_balance mount option

Since restriper kthread starts involuntarily on mount and can suck cpu
and memory bandwidth add a mount option to forcefully skip it.  The
restriper in that case hangs around in paused state and can be resumed
from userspace when it's convenient.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

diff --cc fs/btrfs/ctree.h
Simple merge

diff --cc fs/btrfs/super.c
Simple merge

diff --cc fs/btrfs/volumes.c
Simple merge