f2fs: set raw_super default to NULL to avoid compile warning

Set raw_super default to NULL to avoid the possibly used
uninitialized warning, though we may never hit it in fact.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
1 file changed