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

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.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D37533