lib/printbuf: Heap allocation

This makes printbufs optionally heap allocated: a printbuf initialized
with the PRINTBUF initializer will automatically heap allocate and
resize as needed.

Allocations are done with GFP_KERNEL: code should use e.g.
memalloc_nofs_save()/restore() as needed. Since we do not currently have
memalloc_nowait_save()/restore(), in contexts where it is not safe to
block we provide the helpers

  printbuf_atomic_inc()
  printbuf_atomic_dec()

When the atomic count is nonzero, memory allocations will be done with
GFP_NOWAIT.

On memory allocation failure, output will be truncated. Code that wishes
to check for memory allocation failure (in contexts where we should
return -ENOMEM) should check if printbuf->allocation_failure is set.
Since printbufs are expected to be typically used for log messages and
on a best effort basis, we don't return errors directly.

Other helpers provided by this patch:

 - printbuf_make_room(buf, extra)
   Reallocates if necessary to make room for @extra bytes (not including
   terminating null).

 - printbuf_str(buf)
   Returns a null terminated string equivalent to the contents of @buf.
   If @buf was never allocated (or allocation failed), returns a
   constant empty string.

 - printbuf_exit(buf)
   Releases memory allocated by a printbuf.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

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