sheep/md: add main_fn marker to for_each_object_in_wd()

We should add the main_fn marker to for_each_object_in_wd() because
the function for_each_object_in_wd() could only be called in main
threads at present.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/md: called get_vnode_info() only once in for_each_object_in_wd()

get_vnode_info() should be called only once in for_each_object_in_wd()
(in the main thread) and the result should be passed to
thread_process_path() as a pthread argument.
This avoids lots of calls to get_vnode_info() and is necessary to pass
our thread checker of the sheepdog tracer.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>