dracut-functions.sh: cp with --reflink=auto and --sparse=auto

cp will use COW data copy most of the time. So, it's even better than
hardlink, because we don't have to care about hardlinks, when modifying
files in post.

dracut-functions.sh

@@ -435,7 +435,7 @@ inst_simple() {
         inst "${_src%/*}/.${_src##*/}.hmac" "${target%/*}/.${target##*/}.hmac"
     fi
     ddebug "Installing $_src"
-    cp --sparse=always -pfL "$_src" "${initdir}/$target"
+    cp --reflink=auto --sparse=auto -pfL "$_src" "${initdir}/$target"
 }
 
 # find symlinks linked to given library file