tracing/probes: Fix double addition of offset for @+FOFFSET

tracing/probes: Fix double addition of offset for @+FOFFSET

Since commit 533059281ee5 ("tracing: probeevent: Introduce new argument
 fetching code") wrongly use @offset local variable during the parsing,
the offset value is added twice when dereferencing.
Reset the @offset after setting it in FETCH_OP_FOFFS.

Link: https://lore.kernel.org/all/178217905962.643090.1978577464942171332.stgit@devnote2/
Fixes: 533059281ee5 ("tracing: probeevent: Introduce new argument fetching code")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: stable@vger.kernel.org