We need some overflow checks due to the implementation of TmpFS.
When size_t is 32 bits and off_t is 64 bits, we might overflow our
KBuffer max size and confuse the KBuffer set_size code, causing a VERIFY
failure. Make sure that resulting offset + size will fit in a size_t.
Another constraint, we make sure that the resulting offset + size will
be less than half of the maximum value of a size_t, because we double
the KBuffer size each time we resize it.
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