f2fs: le16_to_cpu for xattr->e_value_size

This patch fixes missing le16 conversion, reported by kbuild test robot.

Fixes: 5f35a2cd

 ("f2fs: Don't update the xattr data that same as the exist")
Reviewed-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>