ext4: fix incorrect function name in comment

ext4: fix incorrect function name in comment

Since commit 6b730a405037 “ext4: hoist ext4_block_write_begin and
replace the __block_write_begin”, the comment should be updated
accordingly from "__block_write_begin" to "ext4_block_write_begin".

Fixes: 6b730a405037 (“ext4: hoist ext4_block_write_begin and replace...")
Signed-off-by: Baolin Liu <liubaolin@kylinos.cn>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://patch.msgid.link/20250812021709.1120716-1-liubaolin12138@163.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>