increase softint_bytes

[Masanobu SAITOH <msaitoh%execsw.org@localhost>]

 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 (msaitoh%execsw.org@localhost
                                 msaitoh%netbsd.org@localhost)