um: fix address-of CMSG_DATA() rvalue in stub

The UML stub takes the address of CMSG_DATA(fd_msg):

    fd_map = (void *)&CMSG_DATA(fd_msg);

CMSG_DATA() is specified by POSIX to return unsigned char *.  Taking
its address is semantically wrong -- the intent is to get a pointer
to the control message data, which is exactly what CMSG_DATA()
already returns.

This happens to compile with glibc because glibc's primary
CMSG_DATA definition accesses a flexible array member:

    #define CMSG_DATA(cmsg) ((cmsg)->__cmsg_data)

An array lvalue can have its address taken, and &array yields the
same address as array.  However, glibc also has an alternative
definition that uses pointer arithmetic (returning an rvalue), and
musl's definition always uses pointer arithmetic:

    /* musl */
    #define CMSG_DATA(cmsg) \
        ((unsigned char *)(((struct cmsghdr *)(cmsg)) + 1))

Taking the address of an rvalue is a hard error in C, so the
current code fails to compile with musl libc.

Remove the erroneous & operator.  The resulting code is correct
regardless of the CMSG_DATA implementation -- it simply assigns the
data pointer, which is what the subsequent code (fd_map[--num_fds])
expects.

No functional change with glibc; fixes the build with musl.

Signed-off-by: Marcel W. Wysocki <maci.stgn@gmail.com>
Link: https://patch.msgid.link/20260215142803.1455757-1-maci.stgn@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: maci0

arch/um/kernel/skas/stub.c

@@ -146,7 +146,7 @@ stub_signal_interrupt(int sig, siginfo_t *info, void *p)
 		/* Receive the FDs */
 		num_fds = 0;
 		fd_msg = msghdr.msg_control;
-		fd_map = (void *)&CMSG_DATA(fd_msg);
+		fd_map = (void *)CMSG_DATA(fd_msg);
 		if (res == iov.iov_len && msghdr.msg_controllen > sizeof(struct cmsghdr))
 			num_fds = (fd_msg->cmsg_len - CMSG_LEN(0)) / sizeof(int);