qa: krbd_fallocate.sh: zero can be munged to truncate

qa: krbd_fallocate.sh: zero can be munged to truncate

The test case is issuing discards that span two objects: the tail of
the first object is truncated, the head of the second object is zeroed.
These discards aren't serial, so there is a race:

  discard i ~ i + 1: truncate i, zero i + 1
  discard i + 1 ~ i + 2: truncate i + 1, zero i + 2

can be executed as

  truncate i + 1, zero i + 2, truncate i, zero i + 1

For object i + 1, the sequence ends up being truncate tail, then zero
head.  This zero op is munged to truncate on the OSD, resulting in size
0 instead of OBJECT_SIZE / 2.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>