ovl: update ->f_iocb_flags when ovl_change_flags() modifies ->f_flags

ovl_change_flags() is an open-coded variant of fs/fcntl.c:setfl() and it
got missed by commit 164f4064 ("keep iocb_flags() result cached in
struct file"); the same change applies there.

Reported-by: Pierre Labastie <pierre.labastie@neuf.fr>
Fixes: 164f4064 ("keep iocb_flags() result cached in struct file")
Cc: <stable@vger.kernel.org> # v6.0
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216738


Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>