f2fs: don't reopen the main block device in f2fs_scan_devices

f2fs: don't reopen the main block device in f2fs_scan_devices

f2fs_scan_devices reopens the main device since the very beginning, which
has always been useless, and also means that we don't pass the right
holder for the reopen, which now leads to a warning as the core super.c
holder ops aren't passed in for the reopen.

Fixes: 3c62be17d4f5 ("f2fs: support multiple devices")
Fixes: 0718afd47f70 ("block: introduce holder ops")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>