repair: helpers for finding in-core inode records w/ free inodes

repair: helpers for finding in-core inode records w/ free inodes

Add the findfirst_free_inode_rec() and next_free_ino_rec() helpers
to assist scanning the in-core inode records for records with at
least one free inode. These will be used to determine what records
are included in the free inode btree.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>