[NET]: Fix skb_truesize_check() assertion

The intent of the assertion in skb_truesize_check() is to check
for skb->truesize being decremented too much by other code,
resulting in a wraparound below zero.

The type of the right side of the comparison causes the compiler to
promote the left side to an unsigned type, despite the presence of an
explicit type cast.  This defeats the check for negativity.

Ensure both sides of the comparison are a signed type to prevent the
implicit type conversion.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>