nvmet: fix a wrong error status returned in error log page

When the command data_len cannot hold all the controller errors,
we should simply return as much errors as we can fit
instead of failing the command.

Signed-off-by: Amit Engel <amit.engel@dell.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>