fuse: capture the unique id of fuse commands being sent

fuse: capture the unique id of fuse commands being sent

The fuse_request_{send,end} tracepoints capture the value of
req->in.h.unique in the trace output.  It would be really nice if we
could use this to match a request to its response for debugging and
latency analysis, but the call to trace_fuse_request_send occurs before
the unique id has been set:

fuse_request_send:    connection 8388608 req 0 opcode 1 (FUSE_LOOKUP) len 107
fuse_request_end:     connection 8388608 req 6 len 16 error -2

(Notice that req moves from 0 to 6)

Move the callsites to trace_fuse_request_send to after the unique id has
been set by introducing a helper to do that for standard fuse_req
requests.  FUSE_FORGET requests are not covered by this because they
appear to be synthesized into the event stream without a fuse_req
object and are never replied to.

Requests that are aborted without ever having been submitted to the fuse
server retain the behavior that only the fuse_request_end tracepoint
shows up in the trace record, and with req==0.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>