btrfs: move checks for DUMMY_ROOT into a helper

Signed-off-by: David Sterba <dsterba@suse.cz>

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

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

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/extent-tree.c
Simple merge

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