vfs: Add SMB_VFS_GET_REAL_FILENAME_AT

vfs: Add SMB_VFS_GET_REAL_FILENAME_AT

In a patchset that I'm working on right now there's the need to call
getrealfilename while the code does have a pathref fsp already
around. Doing the name-based call including non_widelink_open is not
necessary in this case. Start by adding the _at based call to the VFS.

For now, fall back to the name-based call. glusterfs-fuse will in a
future patch be converted to fgetxattr.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>