bcachefs: time_stats: split stats-with-quantiles into a separate structure

Currently, struct time_stats has the optional ability to quantize the
information that it collects.  This is /probably/ useful for callers who
want to see quantized information, but it more than doubles the size of
the structure from 224 bytes to 464.  For users who don't care about
that (e.g. upcoming xfs patches) and want to avoid wasting 240 bytes per
counter, split the two into separate pieces.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>