f2fs: fix the use of XATTR_NODE_OFFSET

This patch fixes the use of XATTR_NODE_OFFSET.

o The offset should not use several MSB bits which are used by marking node
blocks.

o IS_DNODE should handle XATTR_NODE_OFFSET to avoid potential abnormality
during the fsync call.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>