Btrfs: free csums when we're done scrubbing an extent

A user reported scrub taking up an unreasonable amount of ram as it ran.  This
is because we lookup the csums for the extent we're scrubbing but don't free it
up until after we're done with the scrub, which means we can take up a whole lot
of ram.  This patch fixes this by dropping the csums once we're done with the
extent we've scrubbed.  The user reported this to fix their problem.  Thanks,

Reported-and-tested-by: Remco Hosman <remco@hosman.xs4all.nl>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>