fs/pstore: update the backend parameter in pstore module

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>
(cherry picked from commit 42222c2a5d5da7fe4839491d5c44034f40761071)
Orabug: 27994372
Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>