Use directory xattrs for symlinks

Use directory xattrs for symlinks

There is currently a subtle bug in the SA implementation which
can crop up which prevents us from safely using multiple variable
length SAs in one object.

Fortunately, the only existing use case for this are symlinks with
SA based xattrs.  Therefore, until the root cause in the SA code
can be identified and fixed we prevent adding SA xattrs to symlinks.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1468