cifs: Modify struct smb_vol to use kuids and kgids

Add two helper functions get_option_uid and get_option_gid to handle
the work of parsing uid and gids paramaters from the command line and
making kuids and kgids out of them.

Cc: Steve French <smfrench@gmail.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>