media: vb2: clear V4L2_BUF_FLAG_LAST when filling vb2_buffer

V4L2_BUF_FLAG_LAST is a signal from the driver to userspace for buffers
on the capture queue. When userspace queues back a capture buffer with
the flag set, we should clear it.

Otherwise, if userspace restarts streaming after EOS, without
reallocating the buffers, mem2mem devices will erroneously signal EOS
prematurely, as soon as the already flagged buffer is dequeued.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>