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>