kprobes: Use struct_size() in __get_insn_slot()

__get_insn_slot() allocates 'struct kprobe_insn_page' using a custom
structure size calculation macro, KPROBE_INSN_PAGE_SIZE. Replace
KPROBE_INSN_PAGE_SIZE with the struct_size() macro, which is the
preferred way to calculate the size of flexible structures in the kernel
because it handles overflow and makes it easier to change and audit how
flexible structures are allocated across the entire tree.

Link: https://lore.kernel.org/all/20241030-kprobes-fix-counted-by-annotation-v1-2-8f266001fad0@kernel.org/


(Masami modofied this to be applicable without the 1st patch in the series.)

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>