nbd: Avoid off-by-one in long export name truncation

nbd: Avoid off-by-one in long export name truncation

When snprintf returns the same value as the buffer size, the final
byte was truncated to ensure a NUL terminator.  Fortunately, such long
export names are unusual enough, with no real impact other than what
is displayed to the user.

Fixes: 5c86bdf12089
Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200622210355.414941-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
(cherry picked from commit 00d69986da83a74f6f5731c80f8dd09fde95d19a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>