spl: ext: Use partition size for mount

Since commit 9905cae6 ("fs: ext4: check the minimal partition size
to mount"), a valid size needs to be provided when mounting
an ext filesystem. Fix the spl ext driver to use the parition size
instead of 0 when mounting the filesystem.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>