staging: comedi: comedi_buf: absorb __comedi_buf_write_alloc()

This function is only called by comedi_buf_write_alloc(). Absorb it and remove
the unused 'strict' parameter.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>