clarify return value of cifs_convert_flags()

cifs_convert_flags returns 0x20197 in the default case. It's not
immediately evident where that number comes from, so change it
to be an or'ed set of flags. The compiler will boil it down anyway.

(Thanks to Guenter Kukkukk for clarifying the flags).

Signed-off-by: Steve French <sfrench@us.ibm.com>