Btrfs: deal with io_tree->mapping being NULL

I need to add infrastructure to allocate dummy extent buffers for running sanity
tests, and to do this I need to not have to worry about having an
address_mapping for an io_tree, so just fix up the places where we assume that
all io_tree's have a non-NULL ->mapping.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>