admin/heal: Fix deep healing to heal objects under more conditions (#8321)

- Heal if the part.1 is truncated from its original size
- Heal if the part.1 fails while being verified in between
- Heal if the part.1 fails while being at a certain offset

Other cleanups include make sure to flush the HTTP responses
properly from storage-rest-server, avoid using 'defer' to
improve call latency. 'defer' incurs latency avoid them
in our hot-paths such as storage-rest handlers.

Fixes #8319

cmd/xl-v1-healing.go

@@ -191,17 +191,14 @@ func shouldHealObjectOnDisk(xlErr, dataErr error, meta xlMetaV1, quorumModTime t
 		return true
 	}
 	if xlErr == nil {
-		// If xl.json was read fine but there is some problem with the part.N files.
-		if dataErr == errFileNotFound {
+		// If xl.json was read fine but there may be problem with the part.N files.
+		if IsErr(dataErr, []error{
+			errFileNotFound,
+			errFileCorrupt,
+		}...) {
 			return true
 		}
-		if dataErr == errFileUnexpectedSize {
-			return true
-		}
-		if _, ok := dataErr.(HashMismatchError); ok {
-			return true
-		}
-		if quorumModTime != meta.Stat.ModTime {
+		if !quorumModTime.Equal(meta.Stat.ModTime) {
 			return true
 		}
 	}