In zonefs_file_dio_append(), initialize the variable size to 0 to
prevent compilation and static code analizers warning such as:

New smatch warnings:
fs/zonefs/file.c:441 zonefs_file_dio_append() error: uninitialized
symbol 'size'.

The warning is a false positive as size is never actually used
uninitialized.

No functional change.
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/202303191227.GL8Dprbi-lkp@intel.com/Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>