xfs: create agblock bitmap helper to count the number of set regions

In the next patch, the rmap btree repair code will need to estimate the
size of the new ondisk rmapbt.  The size is a function of the number of
records that will be written to disk, and the size of the recordset is
the number of observations made while scanning the filesystem plus the
number of OWN_AG records that will be injected into the rmap btree.

OWN_AG rmap records track the free space btrees, the AGFL, and the new
rmap btree itself.  The repair tool uses a bitmap to record the space
used for all four structures, which is why we need a function to count
the number of set regions.

A reviewer requested that this be pulled into a separate patch with its
own justification, so here it is.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/scrub/agb_bitmap.h

@@ -65,4 +65,9 @@ int xagb_bitmap_set_btblocks(struct xagb_bitmap *bitmap,
 int xagb_bitmap_set_btcur_path(struct xagb_bitmap *bitmap,
 		struct xfs_btree_cur *cur);
 
+static inline uint32_t xagb_bitmap_count_set_regions(struct xagb_bitmap *b)
+{
+	return xbitmap32_count_set_regions(&b->agbitmap);
+}
+
 #endif	/* __XFS_SCRUB_AGB_BITMAP_H__ */

fs/xfs/scrub/bitmap.c

@@ -566,3 +566,17 @@ xbitmap32_test(
 	*len = bn->bn_start - start;
 	return false;
 }
+
+/* Count the number of set regions in this bitmap. */
+uint32_t
+xbitmap32_count_set_regions(
+	struct xbitmap32	*bitmap)
+{
+	struct xbitmap32_node	*bn;
+	uint32_t		nr = 0;
+
+	for_each_xbitmap32_extent(bn, bitmap)
+		nr++;
+
+	return nr;
+}

fs/xfs/scrub/bitmap.h

@@ -62,4 +62,6 @@ int xbitmap32_walk(struct xbitmap32 *bitmap, xbitmap32_walk_fn fn,
 bool xbitmap32_empty(struct xbitmap32 *bitmap);
 bool xbitmap32_test(struct xbitmap32 *bitmap, uint32_t start, uint32_t *len);
 
+uint32_t xbitmap32_count_set_regions(struct xbitmap32 *bitmap);
+
 #endif	/* __XFS_SCRUB_BITMAP_H__ */