dm cache: resolve small nits and improve Documentation

Document passthrough mode, cache shrinking, and cache invalidation.
Also, use strcasecmp() and hlist_unhashed().

Reported-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --cc Documentation/device-mapper/cache.txt
Simple merge

diff --cc drivers/md/dm-cache-policy-mq.c
Simple merge

diff --cc drivers/md/dm-cache-target.c
Simple merge