lisafs: bound Walk and WalkStat response payloads to maxMessageSize

[ibondarenko1]

Problem

WalkHandler and WalkStatHandler size the response payload buffer from the request-controlled req.Path length, and only reject it against math.MaxUint32:

maxPayloadSize := respMetaSize + (len(req.Path) * (*Inode)(nil).SizeBytes())
if maxPayloadSize > math.MaxUint32 {
	// Too much to walk, can't do.
	return 0, unix.EIO
}
payloadBuf := comm.PayloadBuf(uint32(maxPayloadSize))

comm.PayloadBuf for the channel transport returns a slice into a window of maxMessageSize bytes. A maxPayloadSize between maxMessageSize and MaxUint32 therefore indexes past that window.

Sibling precedent

PReadHandler, in the same file, bounds its response payload correctly:

respPayloadLen := respMetaSize + req.Count
if respPayloadLen > c.maxMessageSize {
	return 0, unix.ENOBUFS
}
payloadBuf := comm.PayloadBuf(respPayloadLen)

Change

Bound maxPayloadSize in both Walk handlers against c.maxMessageSize, matching PReadHandler. math is no longer referenced in the file, so its import is dropped. Two one-line changes plus the import removal.

Scope

Hardening. A handler panic of this shape is recovered by Connection.handleMsg and turned into an EREMOTEIO response, so the gofer does not crash, and the request is only reachable from a compromised sentry. This change removes the reliance on that recover() net and makes the two Walk handlers consistent with PReadHandler.

Getdents64Handler has a related but structurally different unbounded re-allocation; it is left for a separate change because its fix is not a one-line bound and would not belong in this minimal cleanup.