ext4: refine extent status tree

This commit refines the extent status tree code.

1) A prefix 'es_' is added to to the extent status tree structure
members.

2) Refactored es_remove_extent() so that __es_remove_extent() can be
used by es_insert_extent() to remove the old extent entry(-ies) before
inserting a new one.

3) Rename extent_status_end() to ext4_es_end()

4) ext4_es_can_be_merged() is define to check whether two extents can
be merged or not.

5) Update and clarified comments.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>