bug#62404: --reflink=auto not falling back appropriately on older kernels

[Paul Eggert]

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.

0001-cp-clarify-commentary.patch
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