xfs_scrub: split the scrub epilogue code into a separate function

xfs_scrub: split the scrub epilogue code into a separate function

Move all the code that updates the internal state in response to a scrub
ioctl() call completion into a separate function.  This will help with
vectorizing scrub calls later on.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>