f2fs: ignore discard return value

f2fs: ignore discard return value

__blkdev_issue_discard() always returns 0, making the error assignment
in __submit_discard_cmd() dead code.

Initialize err to 0 and remove the error assignment from the
__blkdev_issue_discard() call to err. Move fault injection code into
already present if branch where err is set to -EIO.

This preserves the fault injection behavior while removing dead error
handling.

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>