cifs: Add support for mounting Windows 2008 DFS shares

cifs: Add support for mounting Windows 2008 DFS shares

Windows 2008 CIFS servers do not always return PATH_NOT_COVERED when
attempting to access a DFS share.  Therefore, when checking for remote
shares, unconditionally ask for a DFS referral for the UNC (w/out prepath)
before continuing with previous behavior of attempting to access the UNC +
prepath and checking for PATH_NOT_COVERED.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=31092

Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Sean Finney <[email protected]>
Signed-off-by: Pavel Shilovsky <[email protected]>
Signed-off-by: Steve French <[email protected]>