xfs: remove unneeded return value check for *init_cursor()

Since *init_cursor() can always return a valid cursor, the NULL check
in caller is unneeded. So clean them up.
This also keeps the behavior consistent with other callers.

Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>