kill i_dev

One disadvantage of enlarging the size of dev_t is
that struct inode grows. Bad.
We used to have i_dev and i_rdev; today i_rdev has split into
i_rdev, i_bdev and i_cdev. Bad.

It looks like these four fields can be replaced by a single one,
making struct inode smaller. Not bad.

The first step would be to delete the field i_dev, and
all assignments to it, and replace all remaining occurrences
by i_sb->s_dev.

Roughly speaking, the only use of this field is in the stat
system call.

Any objections?

Andries