scm: Return sensible error codes in recv_fds()

Replace "-1" return with errno codes.

ENOMSG and EBADFD were choosen to do not cross with
standard recvmsg() errors (described in its man page).

This patch is need as preparation to making recv_msg()
be able to be non-block, and return EAGAIN and EWOULDBLOCK
in case of no data.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/pie/util-fd.c

@@ -13,8 +13,10 @@
 #ifdef CR_NOGLIBC
 # include "syscall.h"
 # define __sys(foo)	sys_##foo
+# define __sys_err(ret)	ret
 #else
 # define __sys(foo)	foo
+# define __sys_err(ret)	(-errno)
 #endif
 
 #define __memcpy builtin_memcpy

include/common/scm-code.c

@@ -90,7 +90,7 @@ int recv_fds(int sock, int *fds, int nr_fds, void *data, unsigned ch_size)
 
 		ret = __sys(recvmsg)(sock, &fdset.hdr, 0);
 		if (ret <= 0)
-			return ret ? : -1;
+			return ret ? __sys_err(ret) : -ENOMSG;
 
 		cmsg = CMSG_FIRSTHDR(&fdset.hdr);
 		if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS)
@@ -111,7 +111,7 @@ int recv_fds(int sock, int *fds, int nr_fds, void *data, unsigned ch_size)
 		BUG_ON(min_fd > CR_SCM_MAX_FD);
 
 		if (unlikely(min_fd <= 0))
-			return -1;
+			return -EBADFD;
 
 		__memcpy(&fds[i], cmsg_data, sizeof(int) * min_fd);
 		if (data)