ext4: convert inline data to extents when truncate exceeds inline size

Add a check in ext4_setattr() to convert files from inline data storage
to extent-based storage when truncate() grows the file size beyond the
inline capacity. This prevents the filesystem from entering an
inconsistent state where the inline data flag is set but the file size
exceeds what can be stored inline.

Without this fix, the following sequence causes a kernel BUG_ON():

1. Mount filesystem with inode that has inline flag set and small size
2. truncate(file, 50MB) - grows size but inline flag remains set
3. sendfile() attempts to write data
4. ext4_write_inline_data() hits BUG_ON(write_size > inline_capacity)

The crash occurs because ext4_write_inline_data() expects inline storage
to accommodate the write, but the actual inline capacity (~60 bytes for
i_block + ~96 bytes for xattrs) is far smaller than the file size and
write request.

The fix checks if the new size from setattr exceeds the inode's actual
inline capacity (EXT4_I(inode)->i_inline_size) and converts the file to
extent-based storage before proceeding with the size change.

This addresses the root cause by ensuring the inline data flag and file
size remain consistent during truncate operations.

Reported-by:  <syzbot+7de5fe447862fc37576f@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=7de5fe447862fc37576f


Tested-by:  <syzbot+7de5fe447862fc37576f@syzkaller.appspotmail.com>
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
Link: https://patch.msgid.link/20260207043607.1175976-1-kartikey406@gmail.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org