cmsg: Use do/while for error and errorf

Avoid:

  $ make clean && make CFLAGS='-Wpedantic' cmsg.o 2>&1 | head -n5
  rm -f conmon.o cmsg.o ../bin/conmon
  cc -Wpedantic -std=c99 -Os -Wall -Wextra -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include   -c -o cmsg.o cmsg.c
  cmsg.c: In function ‘recvfd’:
  cmsg.c:30:2: warning: ISO C forbids braced-groups within expressions [-Wpedantic]
    ({         \
    ^

Signed-off-by: W. Trevor King <wking@tremily.us>

conmon/cmsg.c

@@ -27,18 +27,18 @@
 #include "cmsg.h"
 
 #define error(s)								\
-	({									\
+	do {									\
 		fprintf(stderr, "nsenter: %s %s\n", s, strerror(errno));	\
 		errno = ECOMM;							\
 		goto err; /* return value */					\
-	})
+	} while (0)
 
 #define errorf(fmt, ...)									\
-	({											\
+	do {											\
 		fprintf(stderr, "nsenter: " fmt ": %s\n", ##__VA_ARGS__, strerror(errno));	\
 		errno = ECOMM;									\
 		goto err; /* return value */							\
-	})
+	} while (0)
 
 /*
  * Sends a file descriptor along the sockfd provided. Returns the return