udf: Support splicing to file

Add explicit support for splicing from pipe to file through
iter_file_splice_write(). Commit 36e2c742 ("fs: don't allow splice
read/write without explicit ops") removed the default .splice_write
operation which effectively removed UDF support for splicing from pipe.

Fixes: 36e2c742

 ("fs: don't allow splice read/write without explicit ops")
Reported-by: kernel test robot <yujie.liu@intel.com>
Link: https://lore.kernel.org/r/202209081443.593ab12-yujie.liu@intel.com


Signed-off-by: Jan Kara <jack@suse.cz>