xfs_db: add alignment filter to freesp command

xfs_db: add alignment filter to freesp command

This adds an alignment filter to the xfs_db freesp command, i.e.

xfs_db> freesp -A 4

will show only 4-block aligned free extents.

This can be useful for finding freespace suitable for allocations
with specific alignment requirements, such as inode allocation.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>