cifs: use fs_context for automounts

[ Upstream commit 9fd29a5b ]

Use filesystem context support to handle dfs links.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>
Stable-dep-of: efc0b0bc ("smb: propagate error code of extract_sharename()")
Signed-off-by: Sasha Levin <sashal@kernel.org>