vfs: stop always overwriting ->mnt_stat in VFS_STATFS

[mjg]

Summary

statfs is called a lot and currently the code does the following:

int
__vfs_statfs(struct mount *mp, struct statfs *sbp)
{
        int error;

        error = mp->mnt_op->vfs_statfs(mp, &mp->mnt_stat);
        if (sbp != &mp->mnt_stat)
                *sbp = mp->mnt_stat;
        return (error);
}

i.e. all callers ovewrrite the content at the same time. Apart from this being a bottleneck, it does not appear to be of any user either. To the best of my grep all fields are either constant since mount/remount or not used by filesystems in the first place, Also note both mount and remount code do VFS_STATFS(mp, &mp->mnt_stat) to populate it.

There are infrequent callers who use it as means to avoid having to allocate a new buffer. I left them alone.

This came when I tried to benchmark statfs after removing mount mtx from the code path (to be posted later).