pcm: express the rewind size limitation logic better

pcm: express the rewind size limitation logic better

There are a few places where the argument of the .rewind or .forward
callback is checked against the same value as returned by .rewindable or
.forwardable. Express this "don't rewind more than rewindable" logic
explicitly, so that the future fixes to the rewindable size can go to
one function instead of two.

While at it, take advantage of the fact that snd_pcm_mmap_avail() cannot
return negative values (except due to integer overflow, which is AFAICS
impossible given the current boundary choice).

Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>