ext4: kunit tests for higher level extent manipulation functions

ext4: kunit tests for higher level extent manipulation functions

Add more kunit tests to cover the high level caller
ext4_map_create_blocks(). We pass flags in a manner that covers
the below function:

1. ext4_ext_handle_unwritten_extents()
  1.1 - Split/Convert unwritten extent to written in endio convtext.
  1.2 - Split/Convert unwritten extent to written in non endio context.
  1.3 - Zeroout tests for the above 2 cases
2. convert_initialized_extent() - Convert written extent to unwritten
   during zero range

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/9d8ad32cb62f44999c0fe3545b44fc3113546c70.1769149131.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>