btrfs: ENOSPC test for a btrfs

btrfs: ENOSPC test for a btrfs

Regression test for an ENOSPC issue when attempting to write to a file in
a filesystem without any data block groups allocated.

The btrfs issue is fixed by the linux kernel patch titled
"Btrfs: don't initialize a space info as full to prevent ENOSPC" and the
regression was introduced by the patch titled
"Btrfs: fix block group ->space_info null pointer dereference".

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>