Allow kern.ipc.maxsockets to be set to current value without error

Normally setting kern.ipc.maxsockets returns EINVAL if the new value
is not greater than the previous value. This can cause spurious
error messages when sysctl.conf is processed multiple times, or when
automation systems try to ensure the sysctl is set to the correct
value. If the value is unchanged, then just do nothing.

Approved by: re (gjb)
PR: 243532
Reviewed by: markj
Sponsored by: Modirum MDPay
Sponsored by: Klara Inc.

(cherry picked from commit c441592a0e1591591665cd037a8a5e9b54675f99)
(cherry picked from commit 4f69c575996e069bfee62af9db2faa3ffa65db71)