freevxfs: Replace one-element array with flexible array member

Replace the deprecated one-element array with a modern flexible array
member in the struct vxfs_dirblk.

Link: https://github.com/KSPP/linux/issues/79


Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20241103121707.102838-3-thorsten.blum@linux.dev


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>