reiserfs: Fix mistake in down_write() conversion

Fix a mistake in commit 0719d343


(reiserfs: Fix reiserfs lock <-> i_xattr_sem dependency inversion)
that has converted a down_write() into a down_read() accidentally.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Christian Kujau <lists@nerdbynature.de>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>