inst_symlink: create parent dir if doesn't exist

If symlink is yet alone in target directory, inst_symlink will fail to
create it.  This is how to reproduce the bug:

 # mkdir /tmp/test
 # ln -s /bin/cp /tmp/test/cp
 # ./dracut -l -f -H /tmp/test.img -I /tmp/test/cp

Result:

[...]

I: *** Including modules done ***
ln: failed to create symbolic link
`/var/tmp/initramfs.Z5isVu//tmp/test/cp': No such file or directory
ln: failed to create symbolic link
`/var/tmp/initramfs.Z5isVu//tmp/test/cp': No such file or directory
I: Wrote /tmp/test.img:

[...]

Following patch creates directory for the symlink if it doesn't exist.

dracut-functions.sh

@@ -545,6 +545,7 @@ inst_symlink() {
             inst "$_realsrc"
         fi
     fi
+    [[ ! -e $initdir/${_target%/*} ]] && inst_dir "${_target%/*}"
     [[ -d ${_target%/*} ]] && _target=$(readlink -f ${_target%/*})/${_target##*/}
     ln -sfn $(convert_abs_rel "${_target}" "${_realsrc}") "$initdir/$_target"
 }