increase softint_bytes

[Masanobu SAITOH]

 Hi, all.

 Some device drivers now allocate a lot of softints.
See:

        http://mail-index.netbsd.org/current-users/2017/11/09/msg032581.html

To avoid this panic, I wrote the following patch:

        http://www.netbsd.org/~msaitoh/softint-20171116-0.dif

Summary:

        - Increase the default size from 8192 bytes to 32768 bytes.
        - Add new option SOFTINT_BYTES to change the value.
        - Add two new read-only sysctls kern.softint.{max,count}

Any comment?

--
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                SAITOH Masanobu (msai...@execsw.org
                                 msai...@netbsd.org)