ceph: no need to wait for transition RDCACHE|RD -> RD

For write when trying to get the Fwb caps we need to keep waiting
on transition from WRBUFFER|WR -> WR to avoid a new WR sync write
from going before a prior buffered writeback happens.

While for read there is no need to wait on transition from
RDCACHE|RD -> RD, and we can just exclude the revoking caps and
force to sync read.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>