xfs: move the inode walk functions further down

Move the inode walk functions further down in the file to limit the
forward declarations to the two walk functions as we add new code that
uses the inode walks.  We'll clean them out later (i.e. after the
deferred inode inactivation series).
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>