linux-user: Fix error handling in flatload.c target_pread()

The flatload.c target_pread() function is supposed to return
0 on success or negative host errnos; however it wasn't
checking lock_user() for failure or returning the errno from
the pread() call. Fix these problems (the first of which is
noted by Coverity).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
