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