linux-user: fix emulation of splice syscall

The second and fourth argument are in/out parameters, store them back
after the syscall.  Also, the fourth argument was mishandled, and EFAULT
handling was missing.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>