fs/pstore: update the backend parameter in pstore module

This patch update the module parameter backend, so it is visible
through /sys/module/pstore/parameters/backend.

For example:
if pstore backend is ramoops, with this patch:
	# cat /sys/module/pstore/parameters/backend
	ramoops
and without this patch:
	# cat /sys/module/pstore/parameters/backend
	(null)

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Mark Salyzyn <salyzyn@android.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>