illumos gate

We normally remove an unlinked node when its last user goes away and the
node becomes inactive. However, we should not do that if the filesystem
is mounted read-only including the case where it has its readonly
property set. The node will remain on the unlinked queue, so it will
not be leaked.

One particular scenario is when we receive an incremental stream into a
mounted read-only filesystem and that stream contains an unlinked file
(still on the unlinked queue). If that file is opened before the
receive and some time later after the receive it becomes inactive we
would remove it and, thus, modify the read-only filesystem. As a
result, the filesystem would diverge from its source and further
incremental receives would not be possible (without forcing a rollback).

Another related scenario, that may or may not be possible depending on an OS / VFS policy,
is when an open file is unlinked, then the filesystem is remounted read-only, and then the file
is closed.