udf: Use unsigned variables for size calculations

To avoid confusing the compiler about possible negative sizes, switch
various size variables that can never be negative from int to u32. Seen
with GCC 13:

../fs/udf/directory.c: In function 'udf_copy_fi':
../include/linux/fortify-string.h:57:33: warning: '__builtin_memcpy' pointer overflow between offset 80 and size [-2147483648, -1] [-Warray-bounds=]
   57 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
...
../fs/udf/directory.c:102:9: note: in expansion of macro 'memcpy'
  102 |         memcpy(&iter->fi, iter->bh[0]->b_data + off, len);
      |         ^~~~~~

Cc: Jan Kara <jack@suse.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230204183427.never.856-kees@kernel.org>