Btrfs: set lockdep class before locking new extent buffer

We've been seeing spurious complaints out of lockdep because the lock class name
changes.  This is happening because when we drop a snapshot we will lock a block
before we've read it in, which sets the lockdep class to whatever the default
is.  Then once we read the thing in we reset the lockdep class to what it is
supposed to be, which blows lockdeps' mind.  This patch should fix the problem,
it appears to be the only place where we do this sort of thing.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>