btrfs: open code RCU for device name

The RCU protected string is only used for a device name, and RCU is used
so we can print the name and eventually synchronize against the rare
device rename in device_list_add().

We don't need the whole API just for that. Open code all the helpers and
access to the string itself.

Notable change is in device_list_add() when the device name is changed,
which is the only place that can actually happen at the same time as
message prints using the device name under RCU read lock.

Previously there was kfree_rcu() which used the embedded rcu_head to
delay freeing the object depending on the RCU mechanism. Now there's
kfree_rcu_mightsleep() which does not need the rcu_head and waits for
the grace period.

Sleeping is safe in this context and as this is a rare event it won't
interfere with the rest as it's holding the device_list_mutex.

Straightforward changes:

- rcu_string_strdup -> kstrdup
- rcu_str_deref -> rcu_dereference
- drop ->str from safe contexts and use rcu_dereference_raw() so it does
  not trigger RCU validators

Historical notes:

Introduced in 606686eeac45 ("Btrfs: use rcu to protect device->name")
with a vague reference of the potential problem described in
https://lore.kernel.org/all/20120531155304.GF11775@ZenIV.linux.org.uk/ .

The RCU protection looks like the easiest and most lightweight way of
protecting the rare event of device rename racing device_list_add()
with a random printk() that uses the device name.

Alternatives: a spin lock would require to protect the printk
anyway, a fixed buffer for the name would be eventually wrong in case
the new name is overwritten when being printed, an array switching
pointers and cleaning them up eventually resembles RCU too much.

The cleanups up to this patch should hide special case of RCU to the
minimum that only the name needs rcu_dereference(), which can be further
cleaned up to use btrfs_dev_name().

Reviewed-by: Daniel Vacek <neelx@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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