[CIFS] fix mount failure with broken pathnames when smb3 mount with mapchars option

When we SMB3 mounted with mapchars (to allow reserved characters : \ / > < * ?
via the Unicode Windows to POSIX remap range) empty paths
(eg when we open "" to query the root of the SMB3 directory on mount) were not
null terminated so we sent garbarge as a path name on empty paths which caused
SMB2/SMB2.1/SMB3 mounts to fail when mapchars was specified.  mapchars is
particularly important since Unix Extensions for SMB3 are not supported (yet)

Signed-off-by: Steve French <smfrench@gmail.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: David Disseldorp <ddiss@suse.de>