virtiofsd: Set up posix_lock hash table for root inode

We setup per inode hash table ->posix_lock to support remote posix locks.
But we forgot to initialize this table for root inode.

Laszlo managed to trigger an issue where he sent a FUSE_FLUSH request for
root inode and lo_flush() found inode with inode->posix_lock NULL and
accessing this table crashed virtiofsd.

May be we can get rid of initializing this hash table for directory
objects completely. But that optimization is for another day.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Message-Id: <20201207195539.GB3107@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>