[stable] mtdchar: fix offset overflow detection

[Ben Hutchings]

Brad Spengler pointed out this fix from 3.6 was missing from the earlier
stable branches:

commit 9c603e53d380459fb62fec7cd085acb0b74ac18f
Author: Linus Torvalds <torvalds at linux-foundation.org>
Date:   Sat Sep 8 12:57:30 2012 -0700

    mtdchar: fix offset overflow detection

It looks cleanly cherry-pickable, although I haven't yet tested this.

Ben.

-- 
Ben Hutchings
The first rule of tautology club is the first rule of tautology club.
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