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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Zheng Yejian <[email protected]>
Cc: Hanjun Guo <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Zhang Jinhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

kernel/acct.c

@@ -350,6 +350,8 @@ static comp_t encode_comp_t(u64 value)
 		exp++;
 	}
 
+	if (exp > (((comp_t) ~0U) >> MANTSIZE))
+		return (comp_t) ~0U;
 	/*
 	 * Clean it up and polish it off.
 	 */