uprobes: sanitiize xol_free_insn_slot()

uprobes: sanitiize xol_free_insn_slot()

1. Clear utask->xol_vaddr unconditionally, even if this addr is not valid,
   xol_free_insn_slot() should never return with utask->xol_vaddr != NULL.

2. Add a comment to explain why do we need to validate slot_addr.

3. Simplify the validation above. We can simply check offset < PAGE_SIZE,
   unsigned underflows are fine, it should work if slot_addr < area->vaddr.

4. Kill the unnecessary "slot_nr >= UINSNS_PER_PAGE" check, slot_nr must
   be valid if offset < PAGE_SIZE.

The next patches will cleanup this function even more.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240929144235.GA9471@redhat.com