A collection of bug fixes designed to make zvol minor functionality more robust

[bprotopopov]

Close the race window in zvol_open() to prevent removal of
zvol_state in the 'first open' code path. Move the call to
check_disk_change() under zvol_state_lock to make sure the
zvol_media_changed() and zvol_revalidate_disk() called by
check_disk_change() are invoked with positive zv_open_count.

Skip opened zvols when removing minors and set private_data
to NULL for zvols that are not in use whose minors are being
removed, to indicate to zvol_open() that the state is gone.
Skip opened zvols when renaming minors to avoid modifying
zv_name that might be in use, e.g. in zvol_ioctl().

Drop zvol_state_lock before calling add_disk() when creating
minors to avoid deadlocks with zvol_open().

Wrap dmu_objset_find() with spl_fstran_mark()/unmark().

[bprotopopov]

This pull request addresses bug #4344.

[behlendorf]

Nice, it's great to see all these edge cases fixed. This LGTM and passed the manual testing I did. It would be ideal if we could add your test script to zconfig.sh, but that could wait for a follow up patch.

[ryao]

This looks good to me too.

[behlendorf]

@bprotopopov if you're happy with this as a final version I'll merge it to master.

[bprotopopov]

Hi, @behlendorf,
I can look into adding the test in to zconfig.sh as well in a follow up patch.
Can you please give me a moment to confirm with my employer if they wanted to include the Copyright message.

[bprotopopov]

Added Copyright lines in the comments.
@behlendorf, I am all set with this commit.

[ryao]

@bprotopopov This needs to be rebased against HEAD.

[behlendorf]

@bprotopopov great, I can take care of the rebase when I apply it to master. I should be able to get it merged today.

[bprotopopov]

@behlendorf appreciate it

[behlendorf]

Merged as:

5428dc5 Make zvol minor functionality more robust