dm-pcache: allow built-in build and rename flush helper

dm-pcache: allow built-in build and rename flush helper

CONFIG_BCACHE is tristate, so dm-pcache can also be built-in.
Switch the Makefile to use obj-$(CONFIG_DM_PCACHE) so the target can be
linked into vmlinux instead of always being a loadable module.

Also rename cache_flush() to pcache_cache_flush() to avoid a global
symbol clash with sunrpc/cache.c's cache_flush().

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>