rgw_file: introduce rgw_mount2 with a bucket name parameter

rgw_file: introduce rgw_mount2 with a bucket name parameter

This explicitly allow mount a specified bucket in librgw.
Originally, mounting a bucket is implemented like a sub-directory
mount in nfs-ganesha FSAL_RGW with lookup_path.
With this change, we allow root_fh to points exactly to the root
of a mounted fs instance, which is a bucket or "/" rather than
always let root_fh points to "/".

Signed-off-by: Gui Hecheng <guihecheng@cmss.chinamobile.com>