Add the ability to write files to ext4 filesystems using the ext4l
driver. This enables the 'save' command to work with ext4l.
The implementation uses the jbd2 journal for crash safety:
- ext4l_write() creates files if needed and writes data
- Journal transactions commit synchronously for durability
- Buffer cache syncs dirty buffers after write operations
The write path consists of the following steps:
1. Lookup or create file via ext4_create()
2. Start journal transaction
3. For each block: get/allocate block, copy data, sync to disk
4. Update inode size and commit transaction
5. Sync all dirty buffers
Add an ext4l_op_ptr() macro to select between a write operation and a
fallback based on CONFIG_EXT4_WRITE, avoiding #ifdefs in fstypes[].
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
2655c75957