affs: initialize fsdata in affs_truncate()

When aops->write_begin() does not initialize fsdata, KMSAN may report
an error passing the latter to aops->write_end().

Fix this by unconditionally initializing fsdata.

Fixes: f2b6a16e ("fs: affs convert to new aops")
Suggested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>