generic/204: tweak reserve pool size

generic/204: tweak reserve pool size

On small block size filesystems, the reserve pool size is kept
constant at 4MB. filesystems with smaller blocks use comparitively
more blocks for indexing metadata (e.g. in the inode and extent
btrees) and so having a higher indirect block usage. Hence we need
to leave the reserve pool at 1024 block and not scale it for a
constant size.

This makes the test pass on a filesystem made with MKFS_OPTIONS="-b
size=1024 -m crc=1".

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>