xpmem: fix mapping of attachment and segment

* Mapping attached part of segment is done at attach time instead of
  make time to work with runtimes (e.g. OpenMPI) xpmem_make-ing the
  entire user-space
* Mapping attached part of segment at attach time can be turned off by
  specifying xpmem_remote_on_demand in kernel argument
* Mapping attachment chooses appropriate page-sizes, i.e., largest
  allowed by memory range and segment page boundary

Fixes: a8696d8 "xpmem: Support large page attachment"
Change-Id: I44663865204036520e5f62fe22b9134ee4629f9b

test/issues/1259/huge_page_xpmem.c

@@ -113,7 +113,7 @@ int main(int argc, char **argv)
 
 		addr.apid = apid;
 		addr.offset = 0;
-		attach = xpmem_attach(addr, pgsize * pgnum, NULL);
+		attach = xpmem_attach(addr, map_size, NULL);
 
 		CHKANDJUMP(attach == (void *)-1, EXIT_FAILURE,
 			   "xpmem_attach failed: %s\n", strerror(errno));