The fragment offset is an 8 byte offset rather than a byte offset, so
the short packet limit is now in fragment block size in order to compare
with the offset value.
The packet flags are in the first 3 bits of the flags/frags byte, and
so after conversion to a uint16 little endian value they are at the
start, not the end of the value - the mask for extracting "more
fragments" is adjusted to match this byte.
Extremely short fragments less than 80 bytes are dropped, but fragments
over 80 bytes are now accepted.
Fixes#5727
Signed-off-by: James Tucker <james@tailscale.com>