xattr dataset prop: change defaults to sa

It's the main recommendation to set xattr=sa
even in man pages, so let's set it by default.

xattr=sa don't use feature flag, so in the worst
case we'll have non-readable xattrs by other
non-openzfs platforms.

Non-overridden default xattr prop of existing pools
will automatically use sa after this commit too.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #15147