btrfs: regression test for ro snapshot creation

btrfs: regression test for ro snapshot creation

Regression test for a btrfs issue where creation of readonly snapshots caused
the filesystem to get into an inconsistent state.

This regression was introduced in the 3.17 kernel and fixed by reverting the
following linux kernel commit:

     Btrfs: race free update of commit root for ro snapshots
     9c3b306e1c9e6be4be09e99a8fe2227d1005effc

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>