cifs: fix error handling when calling cifs_parse_devname

When we allowed separate unc= and prefixpath= mount options, we could
ignore EINVAL errors from cifs_parse_devname. Now that they are
deprecated, we need to check for that as well and fail the mount if it's
malformed.

Also fix a later error message that refers to the unc= option.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>