Revert "dracut-functions: $_x should exist, no need to test."

This reverts commit b2d225a669.

When there's no /sys/dev/block/$2/slaves/*/dev/ exists, $_x will
be assigned with '/sys/dev/block/$2/slaves/*/dev/', this is invalid.
That commit will lead to some warn msg like:
cat: /sys/dev/block/8:16/slaves/*/dev: No such file or directory
cat: /sys/dev/block/8:32/slaves/*/dev: No such file or directory

dracut-functions.sh

@@ -423,6 +423,7 @@ check_block_and_slaves() {
     fi
     [[ -d /sys/dev/block/$2/slaves ]] || return 1
     for _x in /sys/dev/block/$2/slaves/*/dev; do
+        [[ -f $_x ]] || continue
         check_block_and_slaves $1 $(cat "$_x") && return 0
     done
     return 1