Btrfs: fail to mount if we have problems reading the block groups

We don't actually check the return value of btrfs_read_block_groups, so we can
possibly succeed to mount, but then fail to say read the superblock xattr for
selinux which will cause the vfs code to deactivate the super.

This is a problem because in find_free_extent we just assume that we
will find the right space_info for the allocation we want.  But if we
failed to read the block groups, we won't have setup any space_info's,
and we'll hit a NULL pointer deref in find_free_extent.

This patch fixes that problem by checking the return value of
btrfs_read_block_groups, and failing out properly.  I've also added a
check in find_free_extent so if for some reason we don't find an
appropriate space_info, we just return -ENOSPC.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>