UBI: make vid_hdr non-static

Remove 'static' modifier from the 'vid_hdr' local variable. I do not know
how it slipped in, but this is a bug and will break UBI if someone attaches
2 UBI volumes at the same time.

Artem: amended teh commit message, added -stable.

Signed-off-by: Richard Weinberger <rw@linutronix.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@kernel.org