Btrfs: lock the transition from dirty to writeback for an eb

There is a small window where an eb can have no IO bits set on it, which
could potentially result in extent_buffer_under_io() returning false when we
want it to return true, which could result in not fun things happening.  So
in order to protect this case we need to hold the refs_lock when we make
this transition to make sure we get reliable results out of
extent_buffer_udner_io().  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>