Fix an NFS mount attempt where VFS_STATFS() fails.

r353150 added mnt_rootvnode and this seems to have broken NFS mounts when the
VFS_STATFS() called just after VFS_MOUNT() returns an error.
Then the code calls VFS_UNMOUNT(), which calls vflush(), which returns EBUSY.
Then the thread get stuck sleeping on "mntref" in vfs_mount_destroy().
This patch fixes this problem.

Reviewed by: kib, mjg
Differential Revision: https://reviews.freebsd.org/D24022