shm_open2: Implement SHM_GROW_ON_WRITE

Lack of SHM_GROW_ON_WRITE is actively breaking Python's memfd_create tests,
so go ahead and implement it. A future change will make memfd_create always
set SHM_GROW_ON_WRITE, to match Linux behavior and unbreak Python's tests
on -CURRENT.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D25502