Btrfs: fix wrong send_in_progress accounting

Steps to reproduce:
 # mkfs.btrfs -f /dev/sda8
 # mount /dev/sda8 /mnt
 # btrfs sub snapshot -r /mnt /mnt/snap1
 # btrfs sub snapshot -r /mnt /mnt/snap2
 # btrfs send /mnt/snap1 -p /mnt/snap2 -f /mnt/1
 # dmesg

The problem is that we will sort clone roots(include @send_root), it
might push @send_root before thus @send_root's @send_in_progress will
be decreased twice.

Cc: David Sterba <dsterba@suse.cz>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>