This patch wittingly sets BDRV_REQ_NO_FALLBACK and silently ignores
BDRV_REQ_MAY_UNMAP for older librbd versions.

The rationale for this is as follows (citing Ilya Dryomov current RBD
maintainer):

---8<---
a) remove the BDRV_REQ_MAY_UNMAP check in qemu_rbd_co_pwrite_zeroes()
   and as a consequence always unmap if librbd is too old

   It's not clear what qemu's expectation is but in general Write
   Zeroes is allowed to unmap.  The only guarantee is that subsequent
   reads return zeroes, everything else is a hint.  This is how it is
   specified in the kernel and in the NVMe spec.

   In particular, block/nvme.c implements it as follows:

   if (flags & BDRV_REQ_MAY_UNMAP) {
       cdw12 |= (1 << 25);
   }

   This sets the Deallocate bit.  But if it's not set, the device may
   still deallocate:

   """
   If the Deallocate bit (CDW12.DEAC) is set to '1' in a Write Zeroes
   command, and the namespace supports clearing all bytes to 0h in the
   values read (e.g., bits 2:0 in the DLFEAT field are set to 001b)
   from a deallocated logical block and its metadata (excluding
   protection information), then for each specified logical block, the
   controller:
   - should deallocate that logical block;

   ...

   If the Deallocate bit is cleared to '0' in a Write Zeroes command,
   and the namespace supports clearing all bytes to 0h in the values
   read (e.g., bits 2:0 in the DLFEAT field are set to 001b) from
   a deallocated logical block and its metadata (excluding protection
   information), then, for each specified logical block, the
   controller:
   - may deallocate that logical block;
   """

   https://nvmexpress.org/wp-content/uploads/NVM-Express-NVM-Command-Set-Specification-2021.06.02-Ratified-1.pdf



b) set BDRV_REQ_NO_FALLBACK in supported_zero_flags

   Again, it's not clear what qemu expects here, but without it we end
   up in a ridiculous situation where specifying the "don't allow slow
   fallback" switch immediately fails all efficient zeroing requests on
   a device where Write Zeroes is always efficient:

   $ qemu-io -c 'help write' | grep -- '-[zun]'
    -n, -- with -z, don't allow slow fallback
    -u, -- with -z, allow unmapping
    -z, -- write zeroes using blk_co_pwrite_zeroes

   $ qemu-io -f rbd -c 'write -z -u -n 0 1M' rbd:foo/bar
   write failed: Operation not supported
--->8---

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Message-Id: <20210702172356.11574-6-idryomov@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>