crimson/osd: add the delete-head special case of CEPH_OSD_OP_ROLLBACK

crimson/osd: add the delete-head special case of CEPH_OSD_OP_ROLLBACK

This allows to pass `TestLibRBD.TestIOToSnapshot`:

```
[rzarzynski@o06 build]$ CRIMSON_COMPAT=true RBD_FEATURES=1 bin/ceph_test_librbd --gtest_filter=TestLibRBD.TestIOToSnapshot
seed 3954016
Note: Google Test filter = TestLibRBD.TestIOToSnapshot
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from TestLibRBD
[ RUN      ] TestLibRBD.TestIOToSnapshot
using new format!
...
opening testimg@orig
read: 80
write to snapshot returned -30
Read-only file system
num snaps is: 2
expected: 2
snap: orig
snap: written
found orig with size 2097152
found written with size 2097152
num snaps is: 1
expected: 1
snap: orig
found orig with size 2097152
num snaps is: 0
expected: 0
[       OK ] TestLibRBD.TestIOToSnapshot (7510 ms)
[----------] 1 test from TestLibRBD (7510 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (8504 ms total)
[  PASSED  ] 1 test.
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>