btrfs: warn if EXTENT_BUFFER_UPTODATE is set while reading

We recently tracked down a race condition that triggered a read for an
extent buffer with EXTENT_BUFFER_UPTODATE already set.  While this read
was in progress, other concurrent readers would see the UPTODATE bit and
return early as if the read was already complete, making accesses to the
extent buffer conflict with the read operation that was overwriting it.

Add a WARN_ON() to end_bbio_meta_read() for this situation to make
similar races easier to spot in the future.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Tavian Barnes <tavianator@tavianator.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>