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