[RFC] reserved inode fields to use for i_blocks_high

The general consensus is that it is useful to include an extension to the
i_blocks field in conjunction with Takashi Sato's RO_COMPAT_HUGE_FILE change
in "[13/24]ext3 enlarge file size" that changes the i_blocks value to be
stored in fs-blocksize units.  The open question is which field to use.

Takashi suggested using l_i_frag and l_i_fsize together as a 16-bit
i_blocks_high extension, and these were also used in a previous patch
of a similar nature from Goldwyn Rodrigues <[email protected]>
"Pushing ext3 file size limits beyond 2TB".  This has the benefit that
we don't need to conditionalize the use of these fields at all (they are
unused in all ext2 layouts) and this field is already 16 bits in size
(leaving the other 32-bit fields untouched).

Stephen instead suggested using one of the other HURD-reserved fields
on the assumption that HURD won't support > 16TB files until after
they have moved the reserved fields to extended attributes.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.