[kernel][btree] Simplified insert

Refactors the insert path to be simpler, although less optimal in terms
of memory loads and stores. However the simpler code has less
conditions and so can still outperform in many cases.

But: 494059275
Change-Id: I2bacb42070dd2313a84005e4b520916dd5798b0c
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1569473
Reviewed-by: James Robinson <jamesr@google.com>
Commit-Queue: Adrian Danis <adanis@google.com>