log: fixup log_head after relocating global data

When `gd` is relocated during `spl_relocate_stack_gd()` the
doubly-linked circular list in the `log_head` member is broken.

The last element of the list should point back to the initial
`list_head`, but as the initial `list_head` is moved the pointer becomes
stale. As a result the loop in `log_dispatch` would never finish.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>