btrfs: access eb::blocking_writers according to ACCESS_ONCE policies

A nice writeup of the LKMM (Linux Kernel Memory Model) rules for access
once policies can be found here
https://lwn.net/Articles/799218/#Access-Marking%20Policies .

The locked and unlocked access to eb::blocking_writers should be
annotated accordingly, following this:

Writes:

- locked write must use ONCE, may use plain read
- unlocked write must use ONCE

Reads:

- unlocked read must use ONCE
- locked read may use plain read iff not mixed with unlocked read
- unlocked read then locked must use ONCE

There's one difference on the assembly level, where
btrfs_tree_read_lock_atomic and btrfs_try_tree_read_lock used the cached
value and did not reevaluate it after taking the lock. This could have
missed some opportunities to take the lock in case blocking writers
changed between the calls, but the window is just a few instructions
long. As this is in try-lock, the callers handle that.

Signed-off-by: David Sterba <dsterba@suse.com>

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