xfs: Add XFS messages to printk index

In order for end users to quickly react to new issues that come up in
production, it is proving useful to leverage the printk indexing system.

This printk index enables kernel developers to use calls to printk()
with changeable format strings (as they always have; no change of
expectations), while enabling end users to examine format strings to
detect changes.
Since end users are using regular expressions to match messages printed
through printk(), being able to detect changes in chosen format strings
from release to release provides a useful signal to review
printk()-matching regular expressions for any necessary updates.

So that detailed XFS messages are captures by this printk index, this
patch wraps the xfs_<level> and xfs_alert_tag functions.

Signed-off-by: Jonathan Lassoff <jof@thejof.com>
Reviewed-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>