vsprintf: Add support for userspace strings

[Richard Weinberger]

While debugging issues I often add (trace_)printks to strategic positions.
Dealing with user provided string is complicated as an extra buffer a
copy_from_user() is needed.
This adds a new format string to allow direct printing of such strings.

My initial plan was to use %pU but 'U' is already taken, therefore
I used the next letter which comes in mind when one thinks of userpace,
'L'.
The %pL format string works exactly like %s.

[PATCH 1/2] Fix printk() on ERR_PTR()
[PATCH 2/2] vsprintf: Add support for userspace strings
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