Escape any '.' characters in sysctl node names

[asomers]

Summary

ZFS creates some sysctl nodes that include a pool name, and '.' is an
allowed character in pool names. But it's the separator in the sysctl
tree, so it can't be included in a sysctl name. Replace it with "%25".
Handily, "%" is illegal in ZFS pool names, so there's no ambiguity
there.

PR: 257316
MFC after: 3 weeks
Sponsored by: Axcient

Test Plan

manually tested

[martin_lispworks.com]

sys/kern/kern_sysctl.c

771

Is this 1 byte too short (no space for the nul)?

[asomers]

sys/kern/kern_sysctl.c

771

Yep.

[asomers]

• Allow space for the trailing NUL

[freqlabs]

Seems reasonable to me.

sys/kern/kern_sysctl.c

765

Accumulating len is unnecessary since it's just i + 1, which you could do once outside of the loop.