cifs: make a separate function for filling out FILE_UNIX_BASIC_INFO

cifs: make a separate function for filling out FILE_UNIX_BASIC_INFO

The SET_FILE_INFO variant will need to do the same thing here. Break
this code out into a separate function that both variants can call.

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