net: l2: ieee802154: readability: f_ctx -> frag_ctx

The f_ctx variable renamed in this change has a considerable variable
span. This makes the code hard to read as the variable name is neither
defined in the IEEE 802.15.4 standard nor can it be deduced from the
variable name.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>