LU-13802 llite: add read & write switch thresholds

LU-13802 llite: add read & write switch thresholds

The main criteria for switching to from buffered IO to
hybrid is IO size.  This adds that switching.  The correct
size for cutover is not the same for read and write, so we
have separate checks for read and write.

These checks are elaborated on in further patches, adding
different thresholds based on the backing storage type.

Adding the switching thresholds is what really enables
hybrid IO, so we have to adjust a number of tests which
assume buffered IO.

There are a few obscure hang bugs which have been difficult
to track down, and we are past feature freeze, so this patch
now leaves hybrid IO disabled by default.

Signed-off-by: Patrick Farrell <patrick.farrell@oracle.com>
Change-Id: I491cd7b2bdafe8bb2c1a4d692442a62154324bec
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52595
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>