storage: Use the FICLONE ioctl unconditionally on Linux

storage: Use the FICLONE ioctl unconditionally on Linux

According to ioctl_ficlonerange(2)

  These ioctl operations [FICLONE and FICLONERANGE] first
  appeared in Linux 4.5. They were previously known as
  BTRFS_IOC_CLONE and BTRFS_IOC_CLONE_RANGE, and were private
  to Btrfs.

We no longer target any distro that comes with a kernel older
than 4.5, so we can stop looking for the btrfs and xfs
specific versions of the constant and just use the generic
version directly.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>