f2fs: fix a dead loop in f2fs_fiemap()

A dead loop can be triggered in f2fs_fiemap() using the test case
as below:

	...
	fd = open();
	fallocate(fd, 0, 0, 4294967296);
	ioctl(fd, FS_IOC_FIEMAP, fiemap_buf);
	...

It's caused by an overflow in __get_data_block():
	...
	bh->b_size = map.m_len << inode->i_blkbits;
	...
map.m_len is an unsigned int, and bh->b_size is a size_t which is 64 bits
on 64 bits archtecture, type conversion from an unsigned int to a size_t
will result in an overflow.

In the above-mentioned case, bh->b_size will be zero, and f2fs_fiemap()
will call get_data_block() at block 0 again an again.

Fix this by adding a force conversion before left shift.

Signed-off-by: Wei Fang <fangwei1@huawei.com>
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
void f2fs_invalidate_page(struct page *page, unsigned int offset,
							unsigned int length)
{
	struct inode *inode = page->mapping->host;
	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);

	if (inode->i_ino >= F2FS_ROOT_INO(sbi) &&
		(offset % PAGE_SIZE || length != PAGE_SIZE))
		return;

	if (PageDirty(page)) {
		if (inode->i_ino == F2FS_META_INO(sbi)) {
			dec_page_count(sbi, F2FS_DIRTY_META);
		} else if (inode->i_ino == F2FS_NODE_INO(sbi)) {
			dec_page_count(sbi, F2FS_DIRTY_NODES);
		} else {
			inode_dec_dirty_pages(inode);
			remove_dirty_inode(inode);
		}
	}

	/* This is atomic written page, keep Private */
	if (IS_ATOMIC_WRITTEN_PAGE(page))
		return;

	set_page_private(page, 0);
	ClearPagePrivate(page);
}

void f2fs_invalidate_page(struct page *page, unsigned int offset,
							unsigned int length)
{
	struct inode *inode = page->mapping->host;
	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);

	if (inode->i_ino >= F2FS_ROOT_INO(sbi) &&
		(offset % PAGE_SIZE || length != PAGE_SIZE))
		return;

	if (PageDirty(page)) {
		if (inode->i_ino == F2FS_META_INO(sbi)) {
			dec_page_count(sbi, F2FS_DIRTY_META);
		} else if (inode->i_ino == F2FS_NODE_INO(sbi)) {
			dec_page_count(sbi, F2FS_DIRTY_NODES);
		} else {
			inode_dec_dirty_pages(inode);
			remove_dirty_inode(inode);
		}
	}

	/* This is atomic written page, keep Private */
	if (IS_ATOMIC_WRITTEN_PAGE(page))
		return;

	set_page_private(page, 0);
	ClearPagePrivate(page);
}
