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>

bcachefs: mean_and_variance: put struct mean_and_variance_weighted on a diet

The only caller of this code (time_stats) always knows the weights and
whether or not any information has been collected. Pass this
information into the mean and variance code so that it doesn't have to
store that information. This reduces the structure size from 24 to 16
bytes, which shrinks each time_stats counter to 192 bytes from 208.

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

bcachefs: time_stats: add larger units

Filesystems can stay mounted for a very long time, so add some larger
units.

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

bcachefs: pull out time_stats.[ch]

prep work for lifting out of fs/bcachefs/

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>