docs: filesystems: vfs: Convert spaces to tabs

There are bunch of places with 8 spaces, in preparation for correctly
indenting all code snippets (during conversion to RST) change these to
use tabspaces.

This patch is whitespace only.

Convert instances of 8 consecutive spaces to a single tabspace.
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>