[PATCH] proc: inode number fixlet

Ouch, if number taken from IDA is too big, the intent was to signal an
error, not check for overflow and still do overflowing addition.

One still needs 2^28 proc entries to notice this.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>