uprobes: fold xol_take_insn_slot() into xol_get_insn_slot()

After the previous change xol_take_insn_slot() becomes trivial, kill it.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241001142503.GA13633@redhat.com