tarfs: Fix length calculation for file read

[des]

Summary

When the offset is within a block rather than at its beginning, the
length should be clamped to the remaining length of the block, not its
total length.

Reported by: n0ps1ed
MFC after: 1 week

[markj]

sys/fs/tarfs/tarfs_subr.c

431

I think this can be >=? I guess it's harmless as it is.

[des]

rf

[des]

sys/fs/tarfs/tarfs_subr.c

431

Technically this entire if could be dropped and we would just fail the condition of both inner loops and go around the outer loop. It just feels cleaner to have an identifiable section of code for each conceptual case (current block is entirely below the current offset; current block is entirely above the current offset; current block contains the current offset).

[emaste]

sys/fs/tarfs/tarfs_subr.c

431

I was also puzzled by the >, thanks for changing it (even if it has no practical effect).