bcachefs: Don't normalize to pages in btree cache shrinker

bcachefs: Don't normalize to pages in btree cache shrinker

This behavior dates from the early, early days of bcache, and upon
further delving appears to not make any sense. The shrinker only works
in terms of 'objects' of unknown size; normalizing to pages only had the
effect of changing the batch size, which we could do directly - if we
wanted; we probably don't. Normalizing to pages meant our batch size was
very small, which seems to have been keeping us from doing as much
shrinking as we should be under heavy memory pressure; this patch
appears to alleviate some OOMs we've been seeing.

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