cwfs: properly handle 64 bit qid path

for historical reasons, kenfs stores directory entries in pre 9p2000
format with directories having the QPDIR bit 31 set in the qid path.

however, the 64 bit fileserver allows 64 bit qid paths.

given that we do not support pre 9p2000 clients and do not rely on
the QPDIR, but want to keep the block check tags consistent, we will
*INVERT* the QPDIR bit in directory entry qid paths for directories.

this preserves the on-disk semantics (for < 31 bit qmax) but does
not complicate qid generation and recovery. also makes it easy to
convert between directory entry qid and 9p format.

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