bcachefs: time_stats: shrink time_stat_buffer for better alignment

Shrink this percpu object by one array element so that the object size
becomes exactly 512 bytes.  This will lead to more efficient memory use,
hopefully.

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