Btrfs: set truncate block rsv's size

Btrfs: set truncate block rsv's size

While debugging a different issue I noticed that we were always reserving space
when we tried to use our truncate block rsv's.  This is because they didn't have
a ->size value, so use_block_rsv just assumes there is nothing reserved and it
does a reserve_metadata_bytes.  This is because btrfs_check_block_rsv() doesn't
actually add to the size of the block rsv.  That seems to be the right thing to
do so set ->size to the minimum truncate size we need, since we will always only
refill to that size anyway, and this way everything works out correctly.

Signed-off-by: Josef Bacik <josef@redhat.com>