From 1e594bb24d3d08fe4a8afa0fc45a61d6109130fa Mon Sep 17 00:00:00 2001
From: NeilBrown <neilb@suse.de>
Date: Mon, 15 Dec 2014 12:57:00 +1100
Subject: [PATCH] md: tidy up set_bitmap_file

1/ delay setting mddev->bitmap_info.file until 'f' looks
   usable, so we don't have to unset it.
2/ Don't allow bitmap file to be set if bitmap_info.file
   is already set.

Signed-off-by: NeilBrown <neilb@suse.de>
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