librbd: fix AioCompletion race condition

librbd: fix AioCompletion race condition

If block completions finished before all of them where scheduled, the
pending_count would reach 0, and the AioCompletion would be destroyed
since the refcount would also reach 0. To fix this, hold a reference
to the AioCompletion while scheduling more block completions.

This caused a crash like this when running qemu:

1: ceph::__ceph_assert_fail (assertion=<value optimized out>, file=<value optimized out>, line=<value optimized out>, func=<value optimized out>) at common/assert.cc:86
2: Lock (ictx=0x7f83d0001f00, off=5368705024, len=0, buf=0x2fac000 "", c=0x2cebe00) at common/Mutex.h:118
3: add_block_completion (ictx=0x7f83d0001f00, off=5368705024, len=0, buf=0x2fac000 "", c=0x2cebe00) at librbd.cc:159
4: librbd::aio_read (ictx=0x7f83d0001f00, off=5368705024, len=0, buf=0x2fac000 "", c=0x2cebe00) at librbd.cc:1368
5: rbd_aio_rw_vector (bs=<value optimized out>, sector_num=10485752, qiov=<value optimized out>, nb_sectors=<value optimized out>, cb=<value optimized out>, opaque=<value optimized out>, write=0) at block/rbd.c:626
6: qemu_rbd_aio_readv (bs=<value optimized out>, sector_num=<value optimized out>, qiov=<value optimized out>, nb_sectors=<value optimized out>, cb=<value optimized out>, opaque=<value optimized out>) at block/rbd.c:649
7: bdrv_aio_readv (bs=0x2ce92e0, sector_num=10485752, qiov=0x2de08f0, nb_sectors=8, cb=<value optimized out>, opaque=<value optimized out>)

Fixes: #998
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>