proc/sysctl: fix the int overflow for jiffies conversion

do_proc_dointvec_jiffies_conv() uses LONG_MAX/HZ as the max value to
avoid overflow.  But actually the *valp is int type, so it still causes
overflow.

For example,

  echo 2147483647 > ./sys/net/ipv4/tcp_keepalive_time

Then,

  cat ./sys/net/ipv4/tcp_keepalive_time

The output is "-1", it is not expected.

Now use INT_MAX/HZ as the max value instead LONG_MAX/HZ to fix it.

Link: http://lkml.kernel.org/r/1490109532-9228-1-git-send-email-fgao@ikuai8.com
Signed-off-by: Gao Feng <fgao@ikuai8.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>