Btrfs: cleanup: don't check the same thing twice

btrfs_read_fs_root_no_name() already checks if btrfs_root_refs()
is zero and returns ENOENT in this case. There is no need to do
it again in six places.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

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

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

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

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

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