xfs/271: fix test failure on non-reflink filesystems

xfs/271: fix test failure on non-reflink filesystems

This test creates an empty filesystem with rmap btrees enabled, and then
checks that GETFSMAP corresponds (roughly) with what we expect mkfs to
have written to the filesystem.

Unfortunately, the test's calculation for the number of "per-AG
metadata" extents is not quite correct.  For a filesystem with a
refcount btree, the rmapbt and agfl blocks will be reported separately,
but for non-reflink filesystems, GETFSMAP merges the records.

Since this test counts the number of records, fix the calculation.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>