The fdiscard system call, and the file-system-level VOP_DISCARD operation backing it, allow dropping ranges of blocks from files to create sparse files with holes. This is a simple generalization of truncate.

Discard is not currently implemented for FFS; implement it. (For regular files.) It should be possible to do so by generalizing the existing truncate code; this should be not super-difficult, but somewhat messy.

Note that it's important (for both the WAPBL and non-WAPBL cases) to preserve the on-disk invariants needed for successful crash recovery.

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