Btrfs: do not bug_on if we try to cow a free space cache inode

We can just return an error and we'll bail out properly.  We still want to catch
this case to make sure we don't have a bug somewhere, so just warn if this pops
up.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>