kexec: define functions to map and unmap segments

kexec: define functions to map and unmap segments

Implement kimage_map_segment() to enable IMA to map the measurement log
list to the kimage structure during the kexec 'load' stage. This function
gathers the source pages within the specified address range, and maps them
to a contiguous virtual address range.

This is a preparation for later usage.

Implement kimage_unmap_segment() for unmapping segments using vunmap().

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Co-developed-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Signed-off-by: Steven Chen <chenste@linux.microsoft.com>
Acked-by: Baoquan He <bhe@redhat.com>
Tested-by: Stefan Berger <stefanb@linux.ibm.com> # ppc64/kvm
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>