libsa: Disable -Wdangling-pointer for zfs.c.

[jhb]

Summary

GCC 12 warns about a dangling pointer to 'objid' in
zfs_bootenv_initial(). However, this appears to be a false positive
as the pointer to 'objid' is only passed to zfs_lookup_dataset() but
not saved anywhere that outlives the lifetime of the
zfs_bootenv_initial() function.

[emaste]

Is it worth mentioning the false positive in a src comment?

[jhb]

In D37533#853725, @emaste wrote:

Is it worth mentioning the false positive in a src comment?

I think the ZFS code is the vendor OpenZFS code so would be a local diff, so probably not.