btrfs: stripe_alloc: tell a dead tree block from a live one in the metadata RMW audit
The audit reports any older-generation header under a read-modify-write
as a write hole, and counted over six hundred per metadata fill test.
Since runs close at every transaction and freed stripes are reclaimed
whole, most such headers belong to blocks that died with their stripe:
the stripe was claimed fully free, the dead blocks kept their headers,
and a torn parity write over them damages nothing a reader can reach.
Ask the committed extent tree, the witness a degraded read after a
crash would consult, whether the block is still referenced. Count and
report a live one as before (now with its bytenr and reference count),
and count a dead one as meta_rmw_ghost without a warning.
A sector the run has already re-allocated in the current transaction
carries an extent item of the current generation for the new block
while the old header is still on disk (the new write has not landed):
that is not a live block either, so the item generation must match the
header generation. Without that check every single-stripe run of the
current transaction reported its dead predecessors as holes (90 of 135
warnings in one fill test; the other 45 were the raid5 system chunk).
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