ext4: support large block size in ext4_block_write_begin()

Use the EXT4_PG_TO_LBLK() macro to convert folio indexes to blocks to avoid
negative left shifts after supporting blocksize greater than PAGE_SIZE.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Message-ID: <20251121090654.631996-18-libaokun@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: LiBaokun96

fs/ext4/inode.c

@@ -1170,8 +1170,7 @@ int ext4_block_write_begin(handle_t *handle, struct folio *folio,
 	unsigned block_start, block_end;
 	sector_t block;
 	int err = 0;
-	unsigned blocksize = inode->i_sb->s_blocksize;
-	unsigned bbits;
+	unsigned int blocksize = i_blocksize(inode);
 	struct buffer_head *bh, *head, *wait[2];
 	int nr_wait = 0;
 	int i;
@@ -1180,12 +1179,12 @@ int ext4_block_write_begin(handle_t *handle, struct folio *folio,
 	BUG_ON(!folio_test_locked(folio));
 	BUG_ON(to > folio_size(folio));
 	BUG_ON(from > to);
+	WARN_ON_ONCE(blocksize > folio_size(folio));
 
 	head = folio_buffers(folio);
 	if (!head)
 		head = create_empty_buffers(folio, blocksize, 0);
-	bbits = ilog2(blocksize);
-	block = (sector_t)folio->index << (PAGE_SHIFT - bbits);
+	block = EXT4_PG_TO_LBLK(inode, folio->index);
 
 	for (bh = head, block_start = 0; bh != head || !block_start;
 	    block++, block_start = block_end, bh = bh->b_this_page) {