pseudofs: add sysctl to set sbuf buffer size

[dchagin]

Summary

MAXPHYS is to small for the modern program, it is not enough even
for print process memory map.
To avoid this, limit storage size of the sbuf buffer by tunable sysctl.

PR: 237883

[kib]

Why do you need a sysctl ? If bumping, simply increase to some larger but reasonable value, e.g. 2M. IOSIZE_MAX is definitely unreasonable.

[markj]

Seems like the limit could be removed by using an sbuf drain function?

[dchagin]

In D20575#476278, @markj wrote:

Seems like the limit could be removed by using an sbuf drain function?

Some pn_fill() may writes to the sbuf under non-sleepable lock, so better to use some constant which will be > MAXPHYS like kib@ suggested

[dchagin]

limit bufsize by 1Mb