jffs2: avoid Wempty-body warnings

Building with W=1 shows a few warnings for empty macros:

fs/jffs2/scan.c: In function 'jffs2_scan_xattr_node':
fs/jffs2/scan.c:378:66: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
  378 |                 jffs2_sum_add_xattr_mem(s, rx, ofs - jeb->offset);
      |                                                                  ^
fs/jffs2/scan.c: In function 'jffs2_scan_xref_node':
fs/jffs2/scan.c:434:65: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
  434 |                 jffs2_sum_add_xref_mem(s, rr, ofs - jeb->offset);
      |                                                                 ^
fs/jffs2/scan.c: In function 'jffs2_scan_eraseblock':
fs/jffs2/scan.c:893:88: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
  893 |                                 jffs2_sum_add_padding_mem(s, je32_to_cpu(node->totlen));
      |                                                                                        ^

Change all these macros to 'do { } while (0)' statements to avoid the
warnings and make the code a little more robust.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Weinberger <richard@nod.at>