Use function smgrclose() to replace the loop

Hi, Kirill, Junwang,

I made this patch to address the refactor issue in our previous email discussion.

That is, the for loop in function smgrdestroy() and smgrdounlinkall can be replaced with smgrclose().

for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
    smgrsw[reln->smgr_which].smgr_close(reln, forknum);

-->

smgrclose(rels[i]);

Please let me know if you have any questions.

Best Regards,

Steven from Highgo.com