V4L/DVB (6291): Fix: avoid oops on some SMP machines

This workaround fix a bug that happens on some SMP machines. On those machines,
videobuf_iolock is called too soon, before file .mmap handler. This patch calls
the scheduler before iolocking, allowing it to properly call the pending mmap.

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