mountroot: run statfs after mounting devfs

The usual flow for mounting a file system is to VFS_MOUNT() and then
immediately VFS_STATFS().

That's not done in vfs_mountroot_devfs(), which means the
mp->mnt_stat.f_iosize field is not correctly populated, which in turn
causes us to mark valid aio operations as unsafe (because the io size is
set to 0), ultimately causing the aio_test:md_waitcomplete test to fail.

Reviewed by: mckusick
MFC after: 1 week
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D21897