uprobes: pass utask to xol_get_insn_slot() and xol_free_insn_slot()

Add the "struct uprobe_task *utask" argument to xol_get_insn_slot() and
xol_free_insn_slot(), their callers already have it so we can avoid the
unnecessary dereference and simplify the code.

Kill the "tsk" argument of xol_free_insn_slot(), it is always current.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240929144253.GA9487@redhat.com