If you do something like:

  # touch foo
  # tail -f foo &
  # rm foo
  # <take snapshot>
  # <mount snapshot>

you'll panic, because ext3/4 tries to do orphan list processing on the
readonly snapshot device, and:

  kernel: journal commit I/O error
  kernel: Assertion failure in journal_flush_Rsmp_e2f189ce() at journal.c:1356: "!journal->j_checkpoint_transactions"
  kernel: Kernel panic: Fatal exception

for a truly readonly underlying device, it's reasonable and necessary
to just skip orphan list processing.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>