kexec: do a cleanup for function kexec_load

There are a lof of work to be done in function kexec_load, not only for
allocating structs and loading initram, but also for some misc.

To make it more clear, wrap a new function do_kexec_load which is used
to allocate structs and load initram.  And the pre-work will be done in
kexec_load.

Signed-off-by: Minfei Huang <mnfhuang@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Xunlei Pang <xlpang@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>