staging: comedi: comedi_buf: factor out common comedi_buf_write_alloc_* code

The only difference between comedi_buf_write_alloc() and the *_strict()
version is that the *_strict() one will only allocate the chunk if it
can completely fulfill the request.

Factor out the common code and add a flag parameter to indicate the 'strict'
usage. Change the exported functions so they are just wrappers around the
common function.

Cleanup the common function a bit and use the comedi_buf_write_n_available()
helper to determine the number of bytes available.

comedi_buf_write_n_available() is not used outside this module so make it
static. Since the only caller is __comedi_buf_write_alloc(), which will
always have a valid async pointer and already has a memory barrier, we
can remove the unnecessary (async == NULL) test as well as the smp_mb().
Also, the rounding of the sample size can be removed since the caller
does not need it.

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