net: ipv6_fragment: store M-bit in addition to the offset

Currently we only store the fragment offset. But in some cases it might
be necessary to also inspect the M-bit (More Fragment) of all received
fragments.

Modify the semantics of the field to store all the flags, rename the
setter to account for this change, and add a getter for the M-bit.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>