xfs: consolidate incore inode radix tree posteof/cowblocks tags

The clearing of posteof blocks and cowblocks serve the same purpose:
removing speculative block preallocations from inactive files.  We don't
need to burn two radix tree tags on this, so combine them into one.

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