constify more dcache.h inlined helpers.

const struct pointers in commit f0d3b3de ("constify dcache.c
inlined helpers where possible").
This patch allows 'const' in a couple that were added since then.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>