Btrfs-progs: replace find_mount_root from send code

Btrfs-progs: replace find_mount_root from send code

find_mount_root had the problem that it tried to conclude from a file system
path to a mount point, taking the fsid as an indicator. This only works if
no two subvolumes (sharing the same btrfs fsid) are mounted in the same
hierarchy.

Now instead, we're parsing /etc/mtab and look for the longest match.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>