V4L/DVB (7553): videobuf-vmalloc: fix STREAMOFF/STREAMON

There were a small bug on videobuf-vmalloc that were preventing STREAMOFF to
work. The issue is that vmalloc'ed mmaped memory should only be freed after
being sure that there aren't any mmap usage. Otherwise, the memory remap will
stop working, and the userspace won't receive any frames.

This bug were affecting some userspace applications, like tvtime.

After this patch, tvtime started to work again with the drivers that use
videobuf-vmalloc.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>