f2fs: fix sparse warning

make C=2 CHECKFLAGS="-D__CHECK_ENDIAN__"

CHECK   dir.c
dir.c:842:50: warning: cast from restricted __le32
CHECK   node.c
node.c:2759:40: warning: restricted __le32 degrades to integer

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>