media: vb2: be sure to unlock mutex on errors

As reported by smatch:
drivers/media/common/videobuf2/videobuf2-core.c: drivers/media/common/videobuf2/videobuf2-core.c:2159 vb2_mmap() warn: inconsistent returns 'mutex:&q->mmap_lock'.
  Locked on:   line 2148
  Unlocked on: line 2100
               line 2108
               line 2113
               line 2118
               line 2156
               line 2159

There is one error condition that doesn't unlock a mutex.

Fixes: cd26d1c4 ("media: vb2: vb2_mmap: move lock up")
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/common/videobuf2/videobuf2-core.c

@@ -2145,7 +2145,8 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
 	if (length < (vma->vm_end - vma->vm_start)) {
 		dprintk(1,
 			"MMAP invalid, as it would overflow buffer length\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	ret = call_memop(vb, mmap, vb->planes[plane].mem_priv, vma);