Functions that call vm_map_splay_merge sometimes set data fields
(e.g. root->left = NULL) to affect the behavior of that function. This
change stops that data manipulation, and instead calls a pair of
functions, one for the left direction and the other for the right,
with the function called depending whether or not we currently null
the root child in that direction to control the behavior of
vm_map_splay_merge.

Reviewed by: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D22589