kern/nullfs: Add the sysctl to control the vnode caching upon mounting nullfs. (stable/13)

[seigo.tanimura_gmail.com]

Summary

This is an alternative solution to recycle the nullfs vnodes not in use.

• New sysctl OID

• vfs.nullfs.cache_nodes If true (default), a newly mounted nullfs filesystem caches its vnodes after finishing their uses. (0 == v_usecount) If false, a newly mounted nullfs filesystem recycles its vnodes immediately upon finishing their uses. The configuration affects the vnodes in the newly mounted nullfs filesystems only. The vnodes in the nullfs filesystems already mounted are not affected. (NB nullfs does not support updating the mount options.)

Based on: https://reviews.freebsd.org/D44177
Security: FreeBSD-EN-23:18.openzfs
Signed-off-by: Seigo Tanimura <[e-mail]>

[seigo.tanimura_gmail.com]

Target branches: stable/13, releng/13.3, releng/13.2
Backport of: https://reviews.freebsd.org/D44217
Revised diff of: https://reviews.freebsd.org/D44178

[seigo.tanimura_gmail.com]

Unlike https://reviews.freebsd.org/D44178, this fix does not depend on the ZFS fix and can be merged independently from it.

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Reflect https://reviews.freebsd.org/D44217#1008785 .

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Reflect https://reviews.freebsd.org/D44217#1008900.

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Merged to stable/13 as d119f5a194.