Fix single-dive editing oddity

The multi-dive case does fine, but the single-dive case (used when
adding a dive, for example) was somewhat confused between the dive index
(which is the location in the dive array) and the dive number.

Fix this by just passing the dive pointer instead (where NULL means to
use the current dive selection).

Reported-by: Jacco van Koll <jacco.van.koll@gmail.com>
Root-caused-by: Dirk Hohndel <dirk@hohndel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>