ext4: make extent status types exclusive

ext4: make extent status types exclusive

Since we don't add delayed flag in unwritten extents, all of the four
extent status types EXTENT_STATUS_WRITTEN, EXTENT_STATUS_UNWRITTEN,
EXTENT_STATUS_DELAYED and EXTENT_STATUS_HOLE are exclusive now, add
assertion when storing pblock before inserting extent into status tree
and add comment to the status definition.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20240813123452.2824659-11-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>