crimson: migrate off deprecated seastar::smp::count and all_cpus()

crimson: migrate off deprecated seastar::smp::count and all_cpus()

The updated seastar deprecates the global smp::count and smp::all_cpus() in
favour of the per-instance smp::shard_count() and smp::all_shards(). Under the
CI -Werror build these deprecation warnings are fatal.

Replace the uses with the reactor-local free functions
seastar::this_smp_shard_count() and seastar::this_smp_all_shards(), which read
the current smp instance's shard count and shard-id range. crimson runs a
single smp instance and every migrated site executes on a reactor thread (the
alienstore worker threads do not touch these), so the behaviour is unchanged.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>