Allow vm object coalescing to occur in the midst of a vm object when the
OBJ_ONEMAPPING flag is set. In other words, allow recycling of existing
but unused subranges of a vm object when the OBJ_ONEMAPPING flag is set.

Such situations are increasingly common with jemalloc >= 5.0. This
change has the expected effect of reducing the number of vm map entry and
object allocations and increasing the number of superpage promotions.

Reviewed by: kib, markj
Tested by: pho
MFC after: 6 weeks
Differential Revision: https://reviews.freebsd.org/D16501