Export symbols for the full SA API

Export all the symbols for the system attribute (SA) API.  This
allows external module to cleanly manipulate the SAs associated
with a dnode.  Documention for the SA API can be found in the
module/zfs/sa.c source.

This change also removes the zfs_sa_uprade_pre, and
zfs_sa_uprade_post prototypes.  The functions themselves were
dropped some time ago.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>