Use proper helper functions for dive location and for_each_dive

This makes the code use the "dive_has_location()" function rather than
check the longitude and latitude directly.

It also uses "for_each_dive()" rather than open-coding it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>