mount: make setupLoop() work with with Autoclear

setupLoop()'s Autoclear (LO_FLAGS_AUTOCLEAR) will destruct the
loopback device when all associated file descriptors are closed.

However this behavior didn't work before since setupLoop() was
returning a file name. The looppack device was destructed at
the end of the function when LoopParams had Autoclear = true.

Fixes #4969.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

mount/mount_linux.go

@@ -79,14 +79,16 @@ func (m *Mount) Mount(target string) (err error) {
 		// or remount with changed data
 		source := m.Source
 		if losetup {
-			devFile, err := setupLoop(m.Source, LoopParams{
+			loFile, err := setupLoop(m.Source, LoopParams{
 				Readonly:  oflags&unix.MS_RDONLY == unix.MS_RDONLY,
 				Autoclear: true})
 			if err != nil {
 				return err
 			}
+			defer loFile.Close()
+
 			// Mount the loop device instead
-			source = devFile
+			source = loFile.Name()
 		}
 		if err := mountAt(chdir, source, target, m.Type, uintptr(oflags), data); err != nil {
 			return err