kexec: copy only happens before uchunk goes to zero

When loading segments, ubytes is <= mbytes.  When ubytes is exhausted,
there could be remaining mbytes.  Then in the while loop, the buf pointer
advancing with mchunk will causing meaningless reading even though it
doesn't harm.

So let's change to make sure that all of the copying and the rest only
happens before uchunk goes to zero.

Link: https://lkml.kernel.org/r/20240222092119.5602-1-gaoshanliukou@163.com


Signed-off-by: yang.zhang <yang.zhang@hexintek.com>
Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>