Revert "OSD: remove RD flag from CALL ops"

This reverts commit 91e941aef9f55425cc12204146f26d79c444cfae.

We cannot change this op code without breaking compatibility
with old code (client and server).  We'll have to special case
this op code instead.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit d3abd0fe0bb402ff403259d4b1a718a56331fc39)
osd: special case CALL op to not have RD bit effects

In commit 20496b8d2b2c3779a771695c6f778abbdb66d92a we treat a CALL as
different from a normal "read", but we did not adjust the behavior
determined by the RD bit in the op.  We tried to fix that in
91e941aef9f55425cc12204146f26d79c444cfae, but changing the op code breaks
compatibility, so that was reverted.

Instead, special-case CALL in the helper--the only point in the code that
actually checks for the RD bit.  (And fix one lingering user to use that
helper appropriately.)

Fixes: #3731
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit 988a52173522e9a410ba975a4e8b7c25c7801123)