syscall: check inotify() and eventfd() return value

linux-user/syscall.c:555:25: warning: Out of bound memory access (accessed memory precedes memory block)
    target_fd_trans[fd] = trans;
    ~~~~~~~~~~~~~~~~~~~~^~~~~~~

Reported-by: Clang Static Analyzer
Suggested-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
