ext4: refactor split and convert extents

ext4_split_convert_extents() has been historically prone to subtle
bugs and inconsistent behavior due to the way all the various flags
interact with the extent split and conversion process. For example,
callers like ext4_convert_unwritten_extents_endio() and
convert_initialized_extents() needed to open code extent conversion
despite passing CONVERT or CONVERT_UNWRITTEN flags because
ext4_split_convert_extents() wasn't performing the conversion.

Hence, refactor ext4_split_convert_extents() to clearly enforce the
semantics of each flag. The major changes here are:

 * Clearly separate the split and convert process:
   * ext4_split_extent() and ext4_split_extent_at() are now only
     responsible to perform the split.
   * ext4_split_convert_extents() is now responsible to perform extent
     conversion after calling ext4_split_extent() for splitting.
   * This helps get rid of all the MARK_UNWRIT* flags.

 * Clearly enforce the semantics of flags passed to
   ext4_split_convert_extents():

   * EXT4_GET_BLOCKS_CONVERT: Will convert the split extent to written
   * EXT4_GET_BLOCKS_CONVERT_UNWRITTEN: Will convert the split extent to
     unwritten
   * Modify all callers to enforce the above semantics.

 * Use ext4_split_convert_extents() instead of ext4_split_extents()
   in ext4_ext_convert_to_initialized() for uniformity.

 * Now that ext4_split_convert_extents() is handling caching to es, we
   dont need to do it in ext4_split_extent_zeroout().

 * Cleanup all callers open coding the conversion logic. Further, modify
   kuniy tests to pass flags based on the new semantics.

>From an end user point of view, we should not see any changes in
behavior of ext4.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/2084a383d69ceefbaa293b8fcf725365eca0a349.1769149131.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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