orangefs: clean up op_alloc()

fold orangefs_op_initialize() in there, don't bother locking something
nobody else could've seen yet, use kmem_cache_zalloc() instead of
explicit memset()...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>