Fix a long-standing edge-case that would result in a ghosted `zroot' pool

and subsequent headaches caused by multiple pools with the same name.
Specifically, blast away any labels on the designated swap partition.
Problem was when you install to a given layout *with* swap and then turn
around and re-install the same layout *without* swap (we weren't doing a
labelclear for the swap device, so would end up with an "UNAVAIL" status
zroot pool that may only exist in the pool cache).

MFC after:	3 days

0 files changed, 0 insertions, 0 deletions