FIXUP "mds: CDentry: create scrub_info_t"
FIXUP: "mds: CInode: create scrub_info_t"
mds: CDentry: add scrub_info_t infrastructure

In addition to actually having a pointer that is appropriately handled,
we add in an initialization function and a PIN_SCRUBQUEUE.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>
mds: ScrubStack: initial skeleton

This commit introduces the ScrubStack, which is going to be the
data structure handling all of the scrubbing logic. For now it's
just the bare outlines of handling a CDentry stack and some function
declarations that let you see the outlines of its interfaces.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>