BACKPORT: loop: Remove figure_loop_size()

This function was now only used by loop_set_capacity(). Just open code
the remaining code in the caller instead.

Signed-off-by: Martijn Coenen <maco@android.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 0a6ed1b5ff6757f11ad2d57906ceb40488a5ee52)
Bug: 148607611
Change-Id: Icc5ef56a5f0264b9bfa15fbbe8062a74a59d88cb