monitor: fix integer comparison error (clang)

Though the documentation for NLMSG_OK uses an int type for the length
the actual check is based on nlmsghdr->nlmsg_len which is a 32 bit
unsigned integer. Clang was complaining about one call in nlmon.c
because nlmsg_len was int type. Every other usage in nlmon.c uses
a uint32_t, so use that both for consistency and to fix the warning.

monitor/nlmon.c:7998:29: error: comparison of integers of different
		signs: '__u32' (aka 'unsigned int') and 'int'
		[-Werror,-Wsign-compare]
        for (nlmsg = iov.iov_base; NLMSG_OK(nlmsg, nlmsg_len);
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/linux/netlink.h💯24: note: expanded from macro 'NLMSG_OK'
                           (nlh)->nlmsg_len <= (len))

monitor/nlmon.c

@@ -7944,7 +7944,7 @@ static bool nlmon_receive(struct l_io *io, void *user_data)
 	unsigned char buf[8192];
 	unsigned char control[32];
 	ssize_t bytes_read;
-	int nlmsg_len;
+	uint32_t nlmsg_len;
 	int fd;
 
 	fd = l_io_get_fd(io);