bpf: Use get_user() for kprobe_multi symbols and uprobe_multi arrays

copy_user_syms() and bpf_uprobe_multi_link_attach() check whole user
arrays with access_ok() up front, then read them one element at a time
with __get_user(), with other work in between: allocations and
strncpy_from_user() in copy_user_syms(), validation in the uprobe_multi
loop.  Each of those accesses enables and disables user access on its
own, so the up-front check saves nothing.  On x86, __get_user() is even
more expensive than get_user(): it does an lfence, where get_user()
checks the address with a couple of plain instructions.

Use get_user().  The access_ok() checks stay, because they also make a
bad array fail with -EFAULT before anything else is looked up or
allocated.  Update the comment that said they were there for
__get_user().

This is part of getting rid of __get_user() and __put_user() entirely.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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