[PORT FROM R2]Fix spinlock holes in tty_buffer.c

[PORT FROM R2]Fix spinlock holes in tty_buffer.c

BZ: 26164

The tty_buffer_request_room is well protected, while after it return,
it release the port->lock, so the buf.tail is not protected.

Change-Id: I302f78b42c297e16ccf3b93a78254cc3f900326f
Orig-Change-Id: I0e5571803993a436316702b39cf987af1d2a2415
Signed-off-by: Even Xu <feng.xu@intel.com>
Reviewed-on: http://android.intel.com:8080/38084
Reviewed-by: Du, Alek <alek.du@intel.com>
Tested-by: Wang, Zhifeng <zhifeng.wang@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>