fs: Re-implement object layer to remember the fd (#3509)

This patch re-writes FS backend to support shared backend sharing locks for safe concurrent access across multiple servers.

cmd/fallocate_linux.go

@@ -24,8 +24,12 @@ import "syscall"
 // sure that subsequent writes on a file just created will not fail,
 // in addition, file allocation will be contigous on the disk
 func Fallocate(fd int, offset int64, len int64) error {
-	return syscall.Fallocate(fd,
-		1, // FALLOC_FL_KEEP_SIZE
-		offset,
-		len)
+	// No need to attempt fallocate for 0 length.
+	if len == 0 {
+		return nil
+	}
+	// Don't extend size of file even if offset + len is
+	// greater than file size from <bits/fcntl-linux.h>.
+	fallocFLKeepSize := uint32(1)
+	return syscall.Fallocate(fd, fallocFLKeepSize, offset, len)
 }