bdev: add __SPDK_BDEV_MODULE_ONLY to bdev_module.h

We have some cases where libraries need the
spdk_bdev_module structure definition and a couple
of related APIs, but not everything else (i.e.
spdk_bdev, spdk_bdev_io), for purposes of avoiding
abidiff errors.

For example, nvmf creates a dummy spdk_bdev_module,
and then uses it with the spdk_bdev_module_claim_bdev
API to ensure multiple subsystems cannot add the same
bdev as a namespace.

But when nvmf includes bdev_module.h, it pulls in the
spdk_bdev structure definition as well.  This means
when the spdk_bdev structure is modified, it requires
a major version bump since abidiff detects the
difference in the debug info.

Alternatives considered:

* We could add a specific suppression into our abidiff
script for nvmf and struct spdk_bdev, but it would be
a risk (albeit very very small one) that we could
add a real dependency on struct spdk_bdev in the future,
and the suppression would hide the difference.

* We could also break out bdev_module.h into multiple
header files, but the ways of doing that either result
in odd file naming, or modifying every bdev module to
include a new header.

* We could add a public bdev API to expose what the
bdev library needs, but that seemed even more intrusive
than this change. nvmf is kind of abusing the bdev_module
API here, and I'd prefer to not promote that kind of
usage by adding something to the bdev API.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ie8fdef8ea294d005b9ae7934dde49c62748420d1
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/7737


Community-CI: Broadcom CI
Community-CI: Mellanox Build Bot
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>