[PATCH] Fix possible memory corruption in xfs_readlink

[Christoph Hellwig]

We should validate that the value isn't negative in xfs_iformat_*,
although we currently don't do that.  It already verified that it
fits into the XFS_DFORK_DSIZE, which should take care of fitting
into 32-bits.  Adding another explicit check probably won't hurt,
given that XFS_DFORK_DSIZE is calculated dynamically based on the
fork offset.