ANDROID: mnt: Fix null pointer dereference

Fix "ANDROID: mnt: Add filesystem private data to mount points" to only
call update_mnt_data() if the filesystem provides the function. This
avoids a null pointer dereference in do_mount().

Reported-by: youling 257 <youling257@gmail.com>
Signed-off-by: Alistair Delva <adelva@google.com>
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Change-Id: I6aedaa89536ae85e6bc29093f38934c672c8fe42