[media] omap_vout: find_vma() needs ->mmap_sem held

Walking rbtree while it's modified is a Bad Idea(tm); besides,
the result of find_vma() can be freed just as it's getting returned
to caller.  Fortunately, it's easy to fix - just take ->mmap_sem a bit
earlier (and don't bother with find_vma() at all if virtp >= PAGE_OFFSET -
in that case we don't even look at its result).

While we are at it, what prevents VIDIOC_PREPARE_BUF calling
v4l_prepare_buf() -> (e.g) vb2_ioctl_prepare_buf() -> vb2_prepare_buf() ->
__buf_prepare() -> __qbuf_userptr() -> vb2_vmalloc_get_userptr() -> find_vma(),
AFAICS without having taken ->mmap_sem anywhere in process?  The code flow
is bloody convoluted and depends on a bunch of things done by initialization,
so I certainly might've missed something...

Cc: [email protected] [2.6.35]
Signed-off-by: Al Viro <[email protected]>
Cc: Sakari Ailus <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Archit Taneja <[email protected]>
Cc: Prabhakar Lad <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

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