nvmet: add interface to update error-log page

This patch adds nvmet_req based interface to the nvmet-core so that
we can update the error log page. We update error log page in
the request completion path when status is not set to NVME_SC_SUCCESS.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>