cephfs-tool: add lockstat collection and dumping support

cephfs-tool: add lockstat collection and dumping support

When built with CEPH_LOCKSTAT, add --lockstat-dump and --lockstat-threshold
to collect lock contention data during benchmark phases. Start collection
at benchmark begin, reset counters before each read/write phase, and dump
per-iteration/per-phase JSON files (e.g. base_iter1_write.json). Requires
a lockstat-instrumented libcephfs build.

Fixes: https://tracker.ceph.com/issues/76183
Signed-off-by: Edwin Rodriguez <edwin.rodriguez1@ibm.com>