repair: use ir_count for filesystems with sparse inode support

repair: use ir_count for filesystems with sparse inode support

Repair currently assumes each inobt record covers 64 inodes and uses
this value to validate inode counts in the AGI headers and superblock.
This is not always the case with sparse inode support.

Update scan_inobt() to check for sparse inode support and use the new
ir_count field for inode accounting. ir_count contains the total number
of inodes tracked by the record.

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