mm-video-v4l2: Protect buffer access and increase input buffer size

Protect buffer access for below scenarios:

*Increase the scope of buf_lock in free_buffer to avoid access
 of freed buffer for both input and output buffers. Also, add check
 before output buffer access.

*Disallow allocate buffer mode after client has called use buffer.

Allocate additional 512 bytes of memory for input buffers on top of
allocation size as per hardware requirement.

Bug: 64340487
Test: ran POC on sailfish/nyc-mr1-dev
Change-Id: Id699404d6c276c8702826d79b3a6ba9755b3439f