Btrfs: fix deadlock with freeze and sync V2

We can deadlock with freeze right now because we unconditionally start a
transaction in our ->sync_fs() call.  To fix this just check and see if we
have a running transaction to commit.  This saves us from the deadlock
because at this point we'll have the umount sem for the sb so we're safe
from freezes coming in after we've done our check.  With this patch the
freeze xfstests no longer deadlocks.  Thanks,

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