bdev/ocf: take additional reference for ocf_cache

Fixes #1498

When shutting down the application, it was possible to
reference stale ocf_cache pointer. This was the case
when two or more vbdev_ocf devices were based on top
of single cache bdev.

This issue did not occur outside of the shutdown case,
since RPC only allows deletion of the vbdev_ocf.
This erases on disk metadata and next run of the application,
would not detect such vbdev_ocf.

Shutdown meanwhile works different, by first stopping
the instance of running "ocf_mngt_cache" and later detaching
"core" devices (the ones being cached). This prevented
erasing the on disk metadata and allowed for restarted
application to detect vbdev_ocf.
See patch (1292ef24) for details.

Since references to ocf_cache are copied between vbdev_ocf
[see start_cache()], the reference count inside ocf_cache
was limited to original ocf_mngt_cache_start() and
management queue creation. First call into ocf_mngt_cache_stop()
released all references to ocf_cache. Leaving other
vbdev_ocfs pointing to released memory.

This patch works around this issue by increasing ref cnt
on ocf_cache for each vbdev based on top of it.
It allows to call into ocf_mngt_cache_stop(), but not
release the memory for ocf_cache until last vbdev.

Note:
A proper redesign here is in order:
- either rearranging structures to be based around single ocf_cache,
rather than multiple vbdev_ocf instances
- better use of OCF API to reduce book keeping logic in vbdev

There are plans to implement detach/attach in RPC,
so it should be a focus during that effort.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I560a7fbb1c052bf53970e655bdb60803c561a252
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/3574


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>