mounts() moved `first` to the merged fsmeta mount's index whenever
mountFsMeta matched, collapsing the overlay's lowerdir range to the
fsmeta alone and dropping any plain lowers already appended above it.

When fsmerge was added, fsmeta was only ever written for the top
parent of the chain being prepared, so the loop matched at i == 0,
where first already equalled the fsmeta's index and the reassignment
was a no-op. It only mattered once the fsmeta could sit below the top
parent, i.e. a chain extending an already-merged sub-chain. The
snapshotter no longer generates fsmeta itself, so today this only
affects externally supplied fsmeta files.

first only marks the start of the lowerdir range, so it must stay at
the first lower's index.

Signed-off-by: Derek McGowan <derek@mcg.dev>