btrfs-progs: Fix the receive code pathing

btrfs-progs: Fix the receive code pathing

The receive code was not distinguishing properly between the mount root
and the directory to create the received subvolume in.
Also make sure the find_mount_root reports an error if it cannot find
a match at all.

Reported-by: Robert Buhren <robert@robertbuhren.de>
Reported-by: Rory Campbell-Lange <rory@campbell-lange.net>
Reported-by: Stefan Priebe - Profihost AG <s.priebe@profihost.ag>
Signed-off-by: Alex Lyakas <alex.btrfs@zadarastorage.com>
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>