fix(dracut.sh): check kernel zstd support early

Rather than checking late and falling back to "cat" if the kernel
doesn't support zstd decompression, do this early, and use the
fallback code path.

Author: mwilck

dracut.sh

@@ -2350,6 +2350,17 @@ if [[ $create_early_cpio == yes ]]; then
     fi
 fi
 
+if check_kernel_config CONFIG_RD_ZSTD; then
+    DRACUT_KERNEL_RD_ZSTD=yes
+else
+    DRACUT_KERNEL_RD_ZSTD=
+fi
+
+if [[ $compress == $DRACUT_COMPRESS_ZSTD* && ! $DRACUT_KERNEL_RD_ZSTD ]]; then
+    dwarn "dracut: kernel has no zstd support compiled in."
+    compress=
+fi
+
 if [[ $compress && $compress != cat ]]; then
     if ! command -v "${compress%% *}" &> /dev/null; then
         derror "dracut: cannot execute compression command '$compress', falling back to default"
@@ -2360,6 +2371,7 @@ fi
 if ! [[ $compress ]]; then
     # check all known compressors, if none specified
     for i in $DRACUT_COMPRESS_PIGZ $DRACUT_COMPRESS_GZIP $DRACUT_COMPRESS_LZ4 $DRACUT_COMPRESS_LZOP $DRACUT_COMPRESS_ZSTD $DRACUT_COMPRESS_LZMA $DRACUT_COMPRESS_XZ $DRACUT_COMPRESS_LBZIP2 $DRACUT_COMPRESS_BZIP2 $DRACUT_COMPRESS_CAT; do
+        [[ $i != "$DRACUT_COMPRESS_ZSTD" || $DRACUT_KERNEL_RD_ZSTD ]] || continue
         command -v "$i" &> /dev/null || continue
         compress="$i"
         break
@@ -2406,11 +2418,6 @@ case $compress in
         ;;
 esac
 
-if [[ $compress == $DRACUT_COMPRESS_ZSTD* ]] && ! check_kernel_config CONFIG_RD_ZSTD; then
-    dwarn "dracut: kernel has no zstd support compiled in."
-    compress="cat"
-fi
-
 if [[ -n $enhanced_cpio ]]; then
     if [[ $compress == "cat" ]]; then
         # dracut-cpio appends by default, so any ucode remains