f2fs: fix sparse warnings

This patch fixes the below warning.

sparse warnings: (new ones prefixed by >>)

>> fs/f2fs/inode.c:56:23: sparse: restricted __le32 degrades to integer
>> fs/f2fs/inode.c:56:52: sparse: restricted __le32 degrades to integer

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>