dm cache: fix writethrough mode quiescing in cache_map
dm cache: fix writethrough mode quiescing in cache_map

Commit 2ee57d58735 ("dm cache: add passthrough mode") inadvertently
removed the deferred set reference that was taken in cache_map()'s
writethrough mode support.  Restore taking this reference.

This issue was found with code inspection.

Signed-off-by: Mike Snitzer <85e533f2e78486213c4d69f71462342ec0c62734@redhat.com>
Acked-by: Joe Thornber <61d5fad70554ae22d57a37634e04f12f58acc678@redhat.com>
Cc: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org # 3.13+

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs