librbd: initial hooks for client-side, image-extent cache in IO path

[dillaman]

Foundational changes to support a persistent, client-side cache for librbd

[trociny]

@dillaman Observing this:

zhuzha:~/ceph/ceph.upstream/build% RBD_FEATURES=127 ./bin/ceph_test_librbd --gtest_filter=TestLibRBD.SnapRemove2
Note: Google Test filter = TestLibRBD.SnapRemove2
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from TestLibRBD
seed 31855
[ RUN      ] TestLibRBD.SnapRemove2
using new format!
6.67% done
13.33% done
20.00% done
26.67% done
33.33% done
40.00% done
46.67% done
53.33% done
60.00% done
66.67% done
73.33% done
80.00% done
86.67% done
93.33% done
100.00% done
/home/mgolub/ceph/ceph.upstream/src/librbd/operation/SnapshotUnprotectRequest.cc: In function 'int librbd::operation::{anonymous}::C_ScanPoolChildren<I>::send() [with I = librbd::ImageCtx]' thread 7fd39f7fe700 time 2016-08-27 22:49:50.152854
/home/mgolub/ceph/ceph.upstream/src/librbd/operation/SnapshotUnprotectRequest.cc: 67: FAILED assert(image_ctx.owner_lock.is_locked())
 ceph version v11.0.0-1809-g5d7400a (5d7400a8b44d6dd2e482f72e536baec7d766ccaa)
 1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x82) [0x7fd42b968df2]
 2: (()+0x4c523b) [0x7fd42b81323b]
 3: (librbd::AsyncObjectThrottle<librbd::ImageCtx>::start_next_op()+0xb6) [0x7fd42b8279b6]
 4: (librbd::AsyncObjectThrottle<librbd::ImageCtx>::finish_op(int)+0x42) [0x7fd42b827ad2]
 5: (()+0x4c586a) [0x7fd42b81386a]
 6: (Context::complete(int)+0x9) [0x7fd42b5f0c79]
 7: (librados::C_AioComplete::finish(int)+0x1d) [0x7fd42b63b2dd]
 8: (Context::complete(int)+0x9) [0x7fd42b5f0c79]
 9: (Finisher::finisher_thread_entry()+0x21e) [0x7fd42b96806e]
 10: (()+0x80a4) [0x7fd42af160a4]
 10: (()+0x80a4) [0x7fd42af160a4]
 11: (clone()+0x6d) [0x7fd427bb704d]
 NOTE: a copy of the executable, or `objdump -rdS <executable>` is needed to interpret this.
zsh: abort      RBD_FEATURES=127 ./bin/ceph_test_librbd --gtest_filter=TestLibRBD.SnapRemove2

[dillaman]

@trociny sorry about that -- should be resolved now.

[trociny]

lgtm