kexec: make kexec_segment user buffer pointer a union

kexec: make kexec_segment user buffer pointer a union

So far kexec_segment->buf was always a user space pointer as user space
passed the array of kexec_segment structures and kernel copied it.

But with new system call, list of kexec segments will be prepared by
kernel and kexec_segment->buf will point to a kernel memory.

So while I was adding code where I made assumption that ->buf is pointing
to kernel memory, sparse started giving warning.

Make ->buf a union.  And where a user space pointer is expected, access it
using ->buf and where a kernel space pointer is expected, access it using
->kbuf.  That takes care of sparse warnings.

Signed-off-by: Vivek Goyal <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Matthew Garrett <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Dave Young <[email protected]>
Cc: WANG Chao <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>