linux-user/syscall.c: Don't skip stracing for fcntl64 failure case

In an fcntl64 failure path, we were returning directly rather than
simply breaking out of the switch statement. This skips the strace
code for printing the syscall return value, so don't do that.

Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>