jffs2: fix IN_DELETE_SELF on overwriting rename() killing a directory

We don't generate IN_DELETE_SELF on victim of overwriting rename() if
it happens to be a directory.  Trivially fixed by doing to ->i_nlink
what we do ->pino_nlink a couple of lines later in jffs2_rename().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>