btrfs: add readonly flag

setting the readonly flag prevents writes in case an error is detected

Signed-off-by: Arne Jansen <sensille@gmx.net>

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

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

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

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