Thanks for installing that. I found the comments still a bit confusing 
so I pushed the attached; hope it's OK.

It is annoying that in the common case where A is a regular file and B 
does not exist but will be created on the same file system, the syscalls 
start out:

   openat(AT_FDCWD, "B", O_RDONLY|O_PATH|O_DIRECTORY) = -1 ENOENT
   newfstatat(AT_FDCWD, "A", ...}, 0) = 0
   openat(AT_FDCWD, "A", O_RDONLY)        = 3
   newfstatat(3, "", ..., AT_EMPTY_PATH) = 0
   openat(AT_FDCWD, "B", O_WRONLY|O_CREAT|O_EXCL, 0775) = 4
   ioctl(4, BTRFS_IOC_CLONE or FICLONE, 3) = -1 EOPNOTSUPP
   newfstatat(4, "", ..., AT_EMPTY_PATH) = 0

They should be just:

   openat(AT_FDCWD, "A", O_RDONLY)        = 3
   newfstatat(3, "", ..., AT_EMPTY_PATH) = 0
   openat(AT_FDCWD, "B", O_WRONLY|O_CREAT|O_EXCL, 0775) = 4
   ioctl(4, BTRFS_IOC_CLONE or FICLONE, 3) = -1 EOPNOTSUPP
   newfstatat(4, "", ..., AT_EMPTY_PATH) = 0

as there should be no need to stat the source twice, or to try to open 
the destination twice. But this is a performance improvement for another 
day.