orangefs: set up dentry operation at mount time

Set up sb->s_d_op so that all allocated dentries get their d_op set up
at birth time.  This removes lots of code and removes warnings in the
current code where we call d_set_d_op twice on the same dentry.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>