xfs: refactor per-AG inode tagging functions

In preparation for adding another incore inode tree tag, refactor the
code that sets and clears tags from the per-AG inode tree and the tree
of per-AG structures, and remove the open-coded versions used by the
blockgc code.

Note: For reclaim, we now rely on the radix tree tags instead of the
reclaimable inode count more heavily than we used to.  The conversion
should be fine, but the logic isn't 100% identical.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>