When splitting extent, if the second extent can not be dropped, we return
-ENOMEM and use GFP_NOFAIL to preallocate an extent_status outside of
i_es_lock and pass it to __es_remove_extent() to be used as the second
extent. This ensures that __es_remove_extent() is executed successfully,
thus ensuring consistency in the extent status tree. If the second extent
is not undroppable, we simply drop it and return 0. Then retry is no longer
necessary, remove it.

Now, __es_remove_extent() will always remove what it should, maybe more.
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230424033846.4732-6-libaokun1@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>