acct: fix potential integer overflow in encode_comp_t()

In the Linux kernel, the following vulnerability has been resolved:

acct: fix potential integer overflow in encode_comp_t()

The integer overflow is descripted with following codes:

> 317 static comp_t encode_comp_t(u64 value)

> 318 {

> 319 int exp, rnd;

......

> 341 exp <<= MANTSIZE;

> 342 exp += value;

> 343 return exp;

> 344 }

Currently comp_t is defined as type of '__u16', but the variable 'exp' is

type of 'int', so overflow would happen when variable 'exp' in line 343 is

greater than 65535.