inst_symlink: create parent dir if doesn't exist

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.