xfs: simplify usage of the rcur local variable in xfs_refcount_finish_one

Only update rcur when we know the final *pcur value.

Inspired-by: Christoph Hellwig <hch@lst.de>
[djwong: don't leave the caller with a dangling ref]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>