Btrfs: do not hold the file extent leaf locked when adding extent item

For some reason we unlock everything except the leaf we are on, set the path
blocking and then add the extent item for the extent we just finished
writing.  I can't for the life of me figure out why we would want to do
this, and the history doesn't really indicate that there was a real reason
for it, so just remove it.  This will reduce our tree lock contention on
heavy writes.  Thanks,

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