mount2: fix empty directory listings on re-read

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With cmd/mount2, reading a directory more than once returned the correct
entries on the first read but nothing on subsequent reads. Plain `ls`
triggers this: it does lseek(fd, 0, SEEK_SET) to rewind the directory
before a second getdents.

go-fuse v2.9.0 rewinds a directory stream by calling Seekdir on the
FileSeekdirer interface. dirStream did not implement it, so go-fuse
returned ENOTSUP and produced an empty listing on every read after the
first.

This implements Seekdir on dirStream: a rewind to offset 0 resets the
stream to the start, restoring correct listings on re-read. Non-zero
offsets are uncommon for in-memory listings and still return ENOTSUP,
matching go-fuse's own default. A compile-time interface assertion is
added so signature drift on future go-fuse updates is caught at build
time.

Before: second and subsequent reads of a directory returned no entries.
After: directories list correctly on every read.

See: https://github.com/hanwen/go-fuse/issues/549
Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>

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