ocfs2: fix xattr array entry __counted_by error

Commit 2f26f58 ("ocfs2: annotate flexible array members with
__counted_by_le()") started annotating the flexible arrays used by
ocfs2, and now gcc complains about ocfs2_reflink_xattr_header():

  In function ‘fortify_memset_chk’,
      inlined from ‘ocfs2_reflink_xattr_header’ at fs/ocfs2/xattr.c:6365:5:
  include/linux/fortify-string.h:480:25: error: call to ‘__write_overflow_field’ declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]

and it looks like the complaint is valid - even if the actual error
message is somewhat confusing.

The 'last' pointer points to past the end of the counted flex array, but
is used as an actual 'last' entry rather than a 'one-past-last'.

It looks like the code copied and cleared an extra entry (which is
likely harmless in practice), but I don't know ocfs2 at all.  Because
it's also possible that the counted-by annotations are off-by-one, and
so this needs checking by somebody who actually knows ocfs2.

But in the meantime this fixes the build error, and certainly _looks_
sane.

Cc: Dmitry Antipov <dmantipov@yandex.ru>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Heming Zhao <heming.zhao@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: torvalds

fs/ocfs2/xattr.c

@@ -6351,7 +6351,7 @@ static int ocfs2_reflink_xattr_header(handle_t *handle,
 	trace_ocfs2_reflink_xattr_header((unsigned long long)old_bh->b_blocknr,
 					 le16_to_cpu(xh->xh_count));
 
-	last = &new_xh->xh_entries[le16_to_cpu(new_xh->xh_count)];
+	last = &new_xh->xh_entries[le16_to_cpu(new_xh->xh_count)] - 1;
 	for (i = 0, j = 0; i < le16_to_cpu(xh->xh_count); i++, j++) {
 		xe = &xh->xh_entries[i];