Add option to configure inode count for ext filesystems

For a small ext filesystem size of 1GB, the default inode count is
around 2^16, often leading to running out of inodes long before blocks.
Add -n option to zram-init, mapped to mkfs.ext[24] -N option to allow
specifying the desired inode count when making the filesystem.
Merge pull request #2 from qupada/master

Add option to configure inode count for ext filesystems