simple_xattr: permit 0-size extended attributes

If a filesystem uses simple_xattr to support user extended attributes,
LTP setxattr01 and xfstests generic/062 fail with "Cannot allocate
memory": simple_xattr_alloc()'s wrap-around test mistakenly excludes
values of zero size.  Fix that off-by-one (but apparently no filesystem
needs them yet).
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/xattr.c

@@ -843,7 +843,7 @@ struct simple_xattr *simple_xattr_alloc(const void *value, size_t size)
 
 	/* wrap around? */
 	len = sizeof(*new_xattr) + size;
-	if (len <= sizeof(*new_xattr))
+	if (len < sizeof(*new_xattr))
 		return NULL;
 
 	new_xattr = kmalloc(len, GFP_KERNEL);