fix(mount): skip metadata flush for unlinked-while-open files
When a file is unlinked while still open (open-unlink-close pattern),
the synchronous doFlush path recreated the entry on the filer during
close. Check fh.isDeleted before flushing metadata, matching the
existing check in the async flush path.
Remove tests/unlink/14.t from known_failures.txt (all 7 subtests
now pass). Full suite: 235 files, 8803 tests, Result: PASS.