Remove vlog file if bootstrap, syncDir or mmap fails (#1434) (#1436)

The createVlogFile function creates a new vlog file. The function apart
from creating a new file, bootstraps it, syncs it to the disk, and mmaps it.

If any of the three operations fail, we will end up with a file on the
disk but the vlog.maxFid will not be updated.

On the next attempt to create the vlog files, we will get a "File
already exists" error. This commit fixes this issue by removing the file
if createVlogFile encounters any error.

Fixes DGRAPH-1930

(cherry picked from commit dfcca75)

value.go

@@ -1000,14 +1000,26 @@ func (vlog *valueLog) createVlogFile(fid uint32) (*logFile, error) {
 		return nil, errFile(err, lf.path, "Create value log file")
 	}
 
+	removeFile := func() {
+		// Remove the file so that we don't get an error when createVlogFile is
+		// called for the same fid, again. This could happen if there is an
+		// transient error because of which we couldn't create a new file
+		// and the second attempt to create the file succeeds.
+		y.Check(os.Remove(lf.fd.Name()))
+	}
+
 	if err = lf.bootstrap(); err != nil {
+		removeFile()
 		return nil, err
 	}
 
 	if err = syncDir(vlog.dirPath); err != nil {
+		removeFile()
 		return nil, errFile(err, vlog.dirPath, "Sync value log dir")
 	}
+
 	if err = lf.mmap(2 * vlog.opt.ValueLogFileSize); err != nil {
+		removeFile()
 		return nil, errFile(err, lf.path, "Mmap value log file")
 	}