proc_dointvec: write a single value

proc_dointvec: write a single value

The commit 00b7c3395aec3df43de5bd02a3c5a099ca51169f
"sysctl: refactor integer handling proc code"
modified the behaviour of writing to /proc.
Before the commit, write("1\n") to /proc/sys/kernel/printk succeeded. But
now it returns EINVAL.

This commit supports writing a single value to a multi-valued entry.

Signed-off-by: J. R. Okajima <[email protected]>
Reviewed-and-tested-by: WANG Cong <[email protected]>
Signed-off-by: David S. Miller <[email protected]>