[S390] hypfs crashes with invalid mount option.

When an invalid mount option is specified, no root inode is created
for hypfs, hypfs_fill_super() returns with -EINVAL and then
hypfs_kill_super() is called. hypfs_kill_super() does not check if
the root inode has been initialized. This patch adds this check.

Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>