objecter: throttle before looking at lock protected state

objecter: throttle before looking at lock protected state

The take_op_budget() may drop our lock if we are in keep_balanced_budget
mode, so we need to do that _before_ we take references to internal state
that may change out from under us during that time.

This fixes a crash like

./osd/OSDMap.h: In function 'entity_inst_t OSDMap::get_inst(int)':
./osd/OSDMap.h:460: FAILED assert(exists(osd) && is_up(osd))
ceph version 0.22.1 (commit:c6f403a6f441184956e00659ce713eaee7014279)
1: (Objecter::op_submit(Objecter::Op*)+0x6c2) [0x38658854c2]
2: /usr/lib64/librados.so.1() [0x3865855dc9]
3: (RadosClient::aio_write(RadosClient::PoolCtx&, object_t, long,
ceph::buffer::list const&, unsigned long,
RadosClient::AioCompletion*)+0x24b) [0x386585724b]
4: (rados_aio_write()+0x9a) [0x386585741a]
5: /usr/bin/qemu-kvm() [0x45a305]
6: /usr/bin/qemu-kvm() [0x45a430]
7: /usr/bin/qemu-kvm() [0x43bb73]
NOTE: a copy of the executable, or `objdump -rdS <executable>` is
needed to interpret this.
./osd/OSDMap.h: In function 'entity_inst_t OSDMap::get_inst(int)':
./osd/OSDMap.h:460: FAILED assert(exists(osd) && is_up(osd))
ceph version 0.22.1 (commit:c6f403a6f441184956e00659ce713eaee7014279)
1: (Objecter::op_submit(Objecter::Op*)+0x6c2) [0x38658854c2]
2: /usr/lib64/librados.so.1() [0x3865855dc9]
3: (RadosClient::aio_write(RadosClient::PoolCtx&, object_t, long,
ceph::buffer::list const&, unsigned long,
RadosClient::AioCompletion*)+0x24b) [0x386585724b]
4: (rados_aio_write()+0x9a) [0x386585741a]
5: /usr/bin/qemu-kvm() [0x45a305]
6: /usr/bin/qemu-kvm() [0x45a430]
7: /usr/bin/qemu-kvm() [0x43bb73]
NOTE: a copy of the executable, or `objdump -rdS <executable>` is
needed to interpret this.
terminate called after throwing an instance of 'ceph::FailedAssertion'
*** Caught signal (ABRT) ***
ceph version 0.22.1 (commit:c6f403a6f441184956e00659ce713eaee7014279)
1: (sigabrt_handler(int)+0x91) [0x3865922b91]
2: /lib64/libc.so.6() [0x3c0c032a30]
3: (gsignal()+0x35) [0x3c0c0329b5]
4: (abort()+0x175) [0x3c0c034195]
5: (__gnu_cxx::__verbose_terminate_handler()+0x12d) [0x3c110beaad]

Signed-off-by: Sage Weil <sage@newdream.net>