filefrag: accommodate holes when calculating expected values

filefrag: accommodate holes when calculating expected values

Currently, filefrag's "expected physical block" column expects extent
records to be physically adjacent regardless of the amount of logical
block space between the two records.  This means that if we punch a
hole in a file, we get reports like this:

ext:   logical_offset:   physical_offset: length:  expected: flags:
4:     4096..    8343:   57376..  61623:   4248:
5:     8345..   10313:   61625..  63593:   1969:   61624:

Notice how it expects 8345 to map to 61624, and scores this against
the fragmentation of the file.  Flagging this as "unexpected" is
incorrect because the gap in the logical mapping is exactly the same
size as the gap in the physical extents.

Furthermore, this particular mapping leaves the door open to the
optimal mapping -- if a write to block 8344 causes it to be mapped to
61624, the entire range 4096-10313 can be mapped with a single extent.
Until that happens, there's no way to combine extents 4 and 5 because
of the gap in the logical mapping at block 8344.

Therefore, tweak the extent report to account for holes.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>