efi/esrt: Use efi_mem_reserve() and avoid a kmalloc()

We can use the new efi_mem_reserve() API to mark the ESRT table as
reserved forever and save ourselves the trouble of copying the data
out into a kmalloc buffer.

The added advantage is that now the ESRT driver will work across
kexec reboot.

Cc: Peter Jones <pjones@redhat.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>