tracing/mmiotrace: Reset dropped_count in mmio_reset_data()

tracing/mmiotrace: Reset dropped_count in mmio_reset_data()

mmio_reset_data() is called during tracer initialization, reset, and
start. While it resets overrun_detected and prev_overruns, it neglects
to reset dropped_count. Consequently, dropped event counts from prior
tracing sessions persist in dropped_count and corrupt overrun reports
in subsequent runs.

Fix this by explicitly calling atomic_set(&dropped_count, 0) in
mmio_reset_data().

Link: https://patch.msgid.link/178524299122.56416.16277704230639425172.stgit@devnote2
Fixes: 173ed24ee2d6 ("mmiotrace: count events lost due to not recording")
Assisted-by: Antigravity:gemini-3.6-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>