ext4: WARN if a full dir leaf block has only one dentry

The maximum length of a filename is 255 and the minimum block size is 1024,
so it is always guaranteed that the number of entries is greater than or
equal to 2 when do_split() is called. So unless ext4_dx_add_entry() and
make_indexed_dir() or some other functions are buggy, 'split == 0' will
not occur.

Setting 'continued' to 0 in this case masks the problem that the file
system has become corrupted, even though it prevents possible out-of-bounds
access. Hence WARN_ON_ONCE() is used to check if 'split' is 0, and if it is
then warns and returns an error to abort split.

Suggested-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20240823160518.GA424729@mit.edu
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241008121152.3771906-1-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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