nfsd/localio: fix nfsd_file tracepoints to handle NULL rqstp

Otherwise nfsd_file_acquire, nfsd_file_insert_err, and
nfsd_file_cons_err will hit a NULL pointer when they are enabled and
LOCALIO used.

Example trace output (note xid is 0x0 and LOCALIO flag set):
 nfsd_file_acquire: xid=0x0 inode=0000000069a1b2e7
 may_flags=WRITE|LOCALIO ref=1 nf_flags=HASHED|GC nf_may=WRITE
 nf_file=0000000070123234 status=0

Fixes: c63f0e48 ("nfsd: add nfsd_file_acquire_local()")
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

fs/nfsd/trace.h

@@ -1113,7 +1113,7 @@ TRACE_EVENT(nfsd_file_acquire,
 	),
 
 	TP_fast_assign(
-		__entry->xid = be32_to_cpu(rqstp->rq_xid);
+		__entry->xid = rqstp ? be32_to_cpu(rqstp->rq_xid) : 0;
 		__entry->inode = inode;
 		__entry->may_flags = may_flags;
 		__entry->nf_ref = nf ? refcount_read(&nf->nf_ref) : 0;
@@ -1147,7 +1147,7 @@ TRACE_EVENT(nfsd_file_insert_err,
 		__field(long, error)
 	),
 	TP_fast_assign(
-		__entry->xid = be32_to_cpu(rqstp->rq_xid);
+		__entry->xid = rqstp ? be32_to_cpu(rqstp->rq_xid) : 0;
 		__entry->inode = inode;
 		__entry->may_flags = may_flags;
 		__entry->error = error;
@@ -1177,7 +1177,7 @@ TRACE_EVENT(nfsd_file_cons_err,
 		__field(const void *, nf_file)
 	),
 	TP_fast_assign(
-		__entry->xid = be32_to_cpu(rqstp->rq_xid);
+		__entry->xid = rqstp ? be32_to_cpu(rqstp->rq_xid) : 0;
 		__entry->inode = inode;
 		__entry->may_flags = may_flags;
 		__entry->nf_ref = refcount_read(&nf->nf_ref);