linux-user: Fix syscall rt_sigtimedwait() implementation

Implementation of 'rt_sigtimedwait()' in 'syscall.c' uses the
function 'target_to_host_timespec()' to transfer the value of
'struct timespec' from target to host. However, the implementation
doesn't check whether this conversion succeeds and thus can cause
an unaproppriate error instead of the 'EFAULT (Bad address)' which
is supposed to be set if the conversion from target to host fails.

This was confirmed with the LTP test for rt_sigtimedwait:
"/testcases/kernel/syscalls/rt_sigtimedwait/rt_sigtimedwait01.c"
which causes an unapropriate error in test case "test_bad_adress3"
which is run with a bad adress for the 'struct timespec' argument:

FAIL: test_bad_address3 (349): Unexpected failure: EAGAIN/EWOULDBLOCK (11)

The test fails with an unexptected errno 'EAGAIN/EWOULDBLOCK' instead
of the expected EFAULT.

After the changes from this patch, the test case is executed successfully
along with the other LTP test cases for 'rt_sigtimedwait()':

PASS: test_bad_address3 (349): Test passed

Signed-off-by: Filip Bozuta <Filip.Bozuta@syrmia.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200724181651.167819-1-Filip.Bozuta@syrmia.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>