zfs: cherry-pick change from openzfs

Vendor PR:

#16210 FreeBSD: Add const qualifier to members of struct
       opensolaris_utsname

Obtained from: OpenZFS
OpenZFS commit: e2357561b9499296bff758afe4868dbc39735675

Note: this is required by the following up MFC [1]. No functional change.

1. 356be1348dac kernel: Make some compile time constant variables const