rexec: check lseek() return value

Not really needed buy ok.

Fixes: Coverity: 1472769
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/rexec.c

@@ -127,10 +127,13 @@ static void lxc_rexec_as_memfd(char **argv, char **envp, const char *memfd_name)
 		sent = lxc_sendfile_nointr(memfd >= 0 ? memfd : tmpfd, fd, NULL,
 					   st.st_size - bytes_sent);
 		if (sent < 0) {
-			/* Fallback to shoveling data between kernel- and
+			/*
+			 * Fallback to shoveling data between kernel- and
 			 * userspace.
 			 */
-			lseek(fd, 0, SEEK_SET);
+			if (lseek(fd, 0, SEEK_SET) == (off_t) -1)
+				fprintf(stderr, "Failed to seek to beginning of file");
+
 			if (fd_to_fd(fd, memfd >= 0 ? memfd : tmpfd))
 				break;