net: core: failover: enforce mandatory ops and clean up redundant checks

net: core: failover: enforce mandatory ops and clean up redundant checks

The failover framework requires 'ops' to be functional. Currently,
failover_register() allows an instance to be registered with NULL
ops, which leads to inconsistent NULL checks and potential NULL
pointer dereferences in the slave registration paths.

Harden the entry point by requiring non-NULL ops in
failover_register(). This ensures the 'fops' pointer is guaranteed
to be valid for any successfully registered failover instance.
Consequently, remove the now redundant NULL checks for 'fops'
throughout the module to simplify the logic.

Signed-off-by: Zeeshan Ahmad <zeeshanahmad022019@gmail.com>
Link: https://patch.msgid.link/20260302064317.9964-1-zeeshanahmad022019@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>